In a book that I'm reading, a passage form Johann Gottlieb Heineccius is referred to illustrate the difference between codicilli and epistolae (the last paragraph [starting: "unde recte"] is the problematic part, but to give more context I lay here the entire passage):
Codicilli voce nihil Romani denotabant aliud, quam scripturam quamdam ad alios missam. Hinc saepe codicillos pro epistolis ponit Cicero epist. ad Fam. IV 13; VI. 18. ad Quint. Fratr. II. 11. Praecipue tamen id nomen tribuebatur epistolis ad praesentes missis, italice: viglietti. Unde Seneca ep. LV:
"Video te, mi Lucili, quum maxime audio, adeo tecum sum, ut dubitem, an incipiam non epistolas sed codicillos tibi scribere"
Unde recte colligas codicillos, quos deinde imperatores tamquam ultimae voluntatis genus adprobarunt, nihil aliud fuisse quam epistolas scriptas ad haeredes de eo, quod post mortem suam scribentes ab haeredibus fieri vellent. Heineccius lib. II. A. R. p. 543.
My free-translation so far:
For the Romans, Codicilli denoted nothing else but a certain writing that is sent to others. Therefore Cicero sometimes uses Codicilli instead of epistola. Yet, chiefly, this name [codicilli] is given to letters that are sent to those in presence. from this Seneca: [citation that illustrates that is Codicilli and not epistola that being sent to the closed one ("adeo tecum sum")].
But now is the part I'm struggling with and too-far from understanding(which is maybe a citation from another source at least partially):
(from there/where) you are, correctly, collecting codicillos, whom Imperators(?), ["tamquam ultimae voluntatis genus adprobarunt"], nothing but written epistolas to his heirs which after his death, writing (the imporatores?) as they wish to become among the heirs.
Not even sure who are the players in this paragraph or what "tamquam ultimae voluntatis genus" means in the context [and even when it stand-alone], is eo and suam can relate to the same person or should account for 2 different?