How do the different numbering systems for Ambrose’s letters relate to each other?

Question

Background

On p. 27 of Liebeschuetz and Hill’s Ambrose of Milan : Political Letters and Speeches in the series Translated Texts for Historians, vol. 43, they note that:

Latin Text: Otto Faller, Michaela Zelzer CSEL 82.10.1–4. Book X and the Epistulae extra collectionem are in Vo. 3, edited by Michaela Zelzer, whose introductions to Vols 2 and 3 are fundamental to this discussion. Translations: Sister Mary Bayenka [sīc – misspelled by Liebeschuetz and Hill], Saint Ambrose Letters, Fathers of the Church 26, Washington, DC, 1954 (her text is pre-Zelzer) [My emphasis] […] References are to Zelzer’s edition with the number of the old Benedictine/Maurist edition, which is the one printed in PL cited in brackets.

Sister Mary Melchior Beyenka’s edition, p. xv, lists her edition’s numbering with Benedictine enumeration in brackets, and further includes a cross-reference table for Benedictine to Beyenka. Now, on the folio page, there is a note worthy of remark:

Reprinted with corrections 1967
Reprinted 1987

Current assumed numbering systems

This would suggest that the numbering might have been changed. Further, the work L&H reference by Faller and Zelzer is still in copyright, disallowing me to investigate them through online sources. As far as I can tell, there appear to be not two, but three numbering systems used:

1. The Benedictine/Maurist system, ‘which is the one printed in P[atrologia] L[atina]’ (L&H p. 27), in newer editions noted in parentheses.
2. The Zelzer system, in newer editions listed first, which separates the epistulae extrā collectiōnem from the rest of the books.
3. The Beyenka system, 1967 edition, which seems to mostly adhere to Zelzer, but does not separate the epp. ex., but perhaps gives them their own numbers, and does not include (as far as I can tell) epp. listed with a number followed by a letter, e.g. 72A.

Why three systems?

The reason I am suspecting three systems, is this: Though the letters in TTfH seems to follow Beyenka in some cases, there are numerous mismatches:

• TTfH ep. 70 (Maur. 56) does not match the letters numbered 70 in Beyenka. Beyenka 70 is Benedictine 84, and Beyenka 56 is Benedictine 65. However, Beyenka 34 equals Benedictine 56.
• TTfH ep. 71 (Maur. 56A) ≠ Beyenka 71 = Benedictine 54.
  TTfH ep. 71 (Maur. 56A) ≠ Beyenka 56 = Benedictine 65.
  Beyenka does not have any letters’ numbers followed by letters. I cannot find this letter in her collection at all.
• TTfH ep. 72 (Maur. 17) ≠ Beyenka 72 = Benedictine 55.
  TTfH ep. 72 (Maur. 17) ≠ Beyenka 17 = Benedictine 87.
  TTfH ep. 72 (Maur. 17) = Beyenka 7 = Benedictine 17.
  Again, Beyenka does not have any letters’s numbers followed by letters, so the following TTfH ep. 72A, I do not know where to find.
• TTfH ep. 73 (Maur. 18) ≠ Beyenka 73 = Benedictine 39.
  TTfH ep. 73 (Maur. 18) ≠ Beyenka 18 = Benedictine 3.
  TTfH ep. 73 (Maur. 18) = Beyenka 8 = Benedictine 18.

There is a connection, indeed: Beyenka’s edition appears to have its own numbering system, and in her introduction, she does state that the volume is ordered by the function of the persons to whom he wrote:

Because of the wide variety of the subject matter of the letters and the unsatisfactory chronological arrangement of earlier editions and translations, the present volume offers the letters in a new order, which is an adaptation of the classification employed by Palanque. [ɔ: ‘Essai de chronologie Ambrosienne’, Saint Ambroise et l’empire romain, Paris 1933, pp. 480–556.]
— Beyenka, op. cit. v.

The Benedictine numbers match the Benedictine–Maurist system listed in TTfH. But what I cannot figure out, is where the letters for which a number is followed by an alphabet character fall into this system. I am unable to find letters matching this system.

Main question

For these reasons I am looking for an authoritative answer which can cross-list all the epistulae of Ambrose, including the epp. ex. coll. and if relevant, the epistulae which number is followed by a letter, to the Benedictine numbering system. As I already have Beyenka–Benedictine–Beyenka cross-tables, adding her numbering system to an answer is not necessary (unless there are discrepancies that I am not aware of presently, of course). On page 37 of TTfH, they mention that ‘Ep. 72 a (17a), which is not by Ambrose at all, but by Symmachus’ suggests that epistulae numbers followed by a letter indicate that these epistulae are letters to Ambrose in response to the previously numbered epistula. Is this perhaps why I cannot find them in Beyenka?

Reward

I will be adding a bounty to this as soon as it is allowed, as this is very important to me to figure out.

Cross-reference tables

Cross-reference table – Benedictine to Beyenka numbering

BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey
1	1	11	40	21	9	31	74	41	62	51	3	61	5	71	46	81	58
2	15	12	41	22	61	32	75	42	44	52	91	62	6	72	16	82	22
3	18	13	42	23	36	33	76	43	49	53	4	63	59	73	83	83	89
4	19	14	43	24	10	34	51	44	50	54	71	64	77	74	68	84	70
5	32	15	37	25	90	35	52	45	25	55	72	65	56	75	69	85	30
6	33	16	12	26	84	36	53	46	27	56	34	66	87	76	85	86	31
7	20	17	7	27	82	37	54	47	24	57	11	67	57	77	47	87	17
8	21	18	8	28	81	38	55	48	23	58	28	68	88	78	48	88	65
9	38	19	35	29	79	39	73	49	26	59	29	69	78	79	66	89	63
10	39	20	60	30	80	40	2	50	14	60	86	70	45	80	67	90	64
																91	13

Cross-reference table – Beyenka to Benedictine numbering

Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM	Bey	BnM
1	1	11	57	21	8	31	86	41	12	51	37	61	22	71	54	81	28
2	40	12	16	22	82	32	5	42	13	52	35	62	41	72	55	82	27
3	51	13	91	23	48	33	6	43	14	53	36	63	89	73	39	83	73
4	53	14	50	24	47	34	56	44	42	54	37	64	90	74	31	84	26
5	61	15	2	25	45	35	19	45	70	55	38	65	88	75	32	85	76
6	62	16	72	26	49	36	23	46	71	56	65	66	79	76	33	86	60
7	17	17	87	27	46	37	15	47	77	57	67	67	80	77	64	87	66
8	18	18	3	28	58	38	9	48	78	58	81	68	74	78	69	88	68
9	21	19	4	29	59	39	10	49	43	59	63	69	75	79	29	89	83
10	24	20	7	30	85	40	11	50	44	60	20	70	84	80	30	90	25
																91	52

Answer 1

I have found the answer, and hereby provide a table for everyone’s benefit. Zel =  Zelzer; BnM = Benedictine–Maurist; Bey = Beyenka.

Cross-reference table for the enumeration of the letters of Ambrosius (Aurēlius Ambrosius)

Michaela Zelzer, Sister Mary Melchior Beyenka, and Benedictine–Maurist numbers compared.

Zelzer	BnM	Bey	Benedictine– Maurist	Zel	Bey	Beyenka	BnM	Zel
ep. 1	7	20	1	ep. ex. 12	1	1	1	ep. ex. 12
ep. 2	65	56	2	ep. 36	15	2	40	ep. 74
ep. 3	67	57	3	ep. 43	18	2	40	ep. ex. 1a
ep. 4	27	82	4	ep. 5	19	3	51	ep. ex. 11
ep. 5	4	19	5	ep. 56	32	4	53	ep. 25
ep. 6	28	81	6	ep. 57	33	5	61	ep. ex. 2
ep. 7	37	54	7	ep. 1	20	6	62	ep. ex. 3
ep. 8	39	73	8	ep. 55	21	7	17	ep. 72
ep. 9	79	66	9	Gesta conc. Aquil. ep. 1	38	8	18	ep. 73
ep. 10	38	55	10	ep. ex. 4	39	9	21	ep. 75
ep. 11	29	79	10	Gesta conc. Aquil. ep. 2	39	10	24	ep. 30
ep. 12	30	80	11	ep. ex. 5	40	11	57	ep. ex. 10
ep. 13	31	74	12	ep. ex. 6	41	12	16	ep. 52
ep. 14	33	76	13	ep. ex. 9	42	13	91	ep. 53
ep. 15	69	78	14	ep. ex. 8	43	14	50	ep. 28
ep. 16	76	85	15	ep. 51	37	15	2	ep. 36
ep. 17	81	58	16	ep. 52	12	16	72	ep. 69
ep. 18	70	45	17	ep. 72	7	17	87	ep. 47
ep. 19	71	46	17a	ep. 72a	N/A	18	3	ep. 43
ep. 20	77	47	18	ep. 73	8	19	4	ep. 5
ep. 21	34	51	19	ep. 62	35	20	7	ep. 1
ep. 22	35	52	20	ep. 76	60	21	8	ep. 55
ep. 23	36	53	21	ep. 75	9	22	82	ep. 24
ep. 24	82	22	21a	ep. 75a	N/A	23	48	ep. 32
ep. 25	53	4	22	ep. 77	61	24	47	ep. 37
ep. 26	54	71	23	ep. ex. 13	36	25	45	ep. 34
ep. 27	58	28	24	ep. 30	10	26	49	ep. 33
ep. 28	50	14	25	ep. 50	90	27	46	ep. 39
ep. 29	43	49	26	ep. 68	84	28	58	ep. 27
ep. 30	24	10	27	ep. 4	82	29	59	ep. 49
ep. 31	44	50	28	ep. 6	81	30	85	ep. 46
ep. 32	48	23	29	ep. 11	79	31	86	ep. 41
ep. 33	49	26	30	ep. 12	80	32	5	ep. 56
ep. 34	45	25	31	ep. 13	74	33	6	ep. 57
ep. 35	83	89	32	ep. 40	75	34	56	ep. 70
ep. 36	2	15	33	ep. 14	76	35	19	ep. 62
ep. 37	47	24	34	ep. 21	51	36	23	ep. ex. 13
ep. 38	55	72	35	ep. 22	52	37	15	ep. 51
ep. 39	46	27	36	ep. 23	53	38	9	Gesta conc. Aquil. ep. 1
ep. 40	32	75	37	ep. 7	54	39	10	ep. ex. 4
ep. 41	86	31	38	ep. 10	55	39	10	Gesta conc. Aquil. ep. 2
ep. 42	88	65	39	ep. 8	73	40	11	ep. ex. 5
ep. 43	3	18	40	ep. 74	2	41	12	ep. ex. 6
ep. 44	68	88	40	ep. ex. 1a	2	42	13	ep. ex. 9
ep. 45	52	91	41	ep. ex. 1	62	43	14	ep. ex. 8
ep. 46	85	30	41a	ep. ex. Sirici	N/A	44	42	ep. ex. 15
ep. 47	87	17	42	ep. ex. 15	44	45	70	ep. 18
ep. 48	66	87	43	ep. 29	49	46	71	ep. 19
ep. 49	59	29	44	ep. 31	50	47	77	ep. 20
ep. 50	25	90	45	ep. 34	25	48	78	ep. 66
ep. 51	15	37	46	ep. 39	27	49	43	ep. 29
ep. 52	16	12	47	ep. 37	24	50	44	ep. 31
ep. 53	91	13	48	ep. 32	23	51	34	ep. 21
ep. 54	64	77	49	ep. 33	26	52	35	ep. 22
ep. 55	8	21	50	ep. 28	14	53	36	ep. 23
ep. 56	5	32	51	ep. ex. 11	3	54	37	ep. 7
ep. 57	6	33	52	ep. 45	91	55	38	ep. 10
ep. 58	60	86	53	ep. 25	4	56	65	ep. 2
ep. 59	84	70	54	ep. 26	71	57	67	ep. 3
ep. 60	90	64	55	ep. 38	72	58	81	ep. 17
ep. 61	89	63	56	ep. 70	34	59	63	ep. ex. 14
ep. 62	19	35	56a	ep. 71	N/A	60	20	ep. 76
ep. 63	73	83	57	ep. ex. 10	11	61	22	ep. 77
ep. 64	74	68	58	ep. 27	28	62	41	ep. ex. 1
ep. 65	75	69	59	ep. 49	29	63	89	ep. 61
ep. 66	78	48	60	ep. 58	86	64	90	ep. 60
ep. 67	80	67	61	ep. ex. 2	5	65	88	ep. 42
ep. 68	26	84	62	ep. ex. 3	6	66	79	ep. 9
ep. 69	72	16	63	ep. ex. 14	59	67	80	ep. 67
ep. 70	56	34	64	ep. 54	77	68	74	ep. 64
ep. 71	56a	N/A	65	ep. 2	56	69	75	ep. 65
ep. 72	17	7	66	ep. 48	87	70	84	ep. 59
ep. 72a	17a	N/A	67	ep. 3	57	71	54	ep. 26
ep. 73	18	8	68	ep. 44	88	72	55	ep. 38
ep. 74	40	2	69	ep. 15	78	73	39	ep. 8
ep. 75	21	9	70	ep. 18	45	74	31	ep. 13
ep. 75a	21a	N/A	71	ep. 19	46	75	32	ep. 40
ep. 76	20	60	72	ep. 69	16	76	33	ep. 14
ep. 77	22	61	73	ep. 63	83	77	64	ep. 54
ep. ex. 1	41	62	74	ep. 64	68	78	69	ep. 15
ep. ex. 1a	40	2	75	ep. 65	69	79	29	ep. 11
ep. ex. 2	61	5	76	ep. 16	85	80	30	ep. 12
ep. ex. 3	62	6	77	ep. 20	47	81	28	ep. 6
ep. ex. 4	10	39	78	ep. 66	48	82	27	ep. 4
ep. ex. 5	11	40	79	ep. 9	66	83	73	ep. 63
ep. ex. 6	12	41	80	ep. 67	67	84	26	ep. 68
ep. ex. 7	N/A	N/A	81	ep. 17	58	85	76	ep. 16
ep. ex. 8	14	43	82	ep. 24	22	86	60	ep. 58
ep. ex. 9	13	42	83	ep. 35	89	87	66	ep. 48
ep. ex. 10	57	11	84	ep. 59	70	88	68	ep. 44
ep. ex. 11	51	3	85	ep. 46	30	89	83	ep. 35
ep. ex. 12	1	1	86	ep. 41	31	90	25	ep. 50
ep. ex. 13	23	36	87	ep. 47	17	91	52	ep. 45
ep. ex. 14	63	59	88	ep. 42	65	N/A	56a	ep. 71
ep. ex. Sirici	41a	N/A	89	ep. 61	63	N/A	17a	ep. 72a
ep. ex. 15	42	44	90	ep. 60	64	N/A	21a	ep. 75a
Gesta conc. Aquil. ep. 1	9	38	91	ep. 53	13	N/A	N/A	ep. ex. 7
Gesta conc. Aquil. ep. 2	10	39	ante ep. 9	Gesta conc. Aquil. Acta	N/A	N/A	41a	ep. ex. Sirici
Gesta conc. Aquil. Acta	ante ep. 9	N/A	N/A	ep. ex. 7	N/A	N/A	ante ep. 9	Gesta conc. Aquil. Acta

Source: Beyenka’s edition as above cross-checked with Corpus scriptorum ecclesiasticorum latinorum vol. LXXXII pars X, pp. XIV–XVIII. To assure the utmost correctness, I used a spreadsheet to cross-check the two tables against each other. Nevertheless, should anyone find any errors, please do comment below, so this answer can be a definitive answer to be found online.