One pants? One scissors? Oh, no way! This is a follow-up to the @brianpck's question How do I specify how many “litterae” or “castra” there are?, and the accepted answer was to use distributive numerals with the pluralia tantum nouns, which are morphologically plural even when their denotate is a single object. The answser did not provide any exceptions for expressing the quantity of one such a thing, and the answerer even confirmed that a distributive form is used with a single castra or litterae in my follow-up comment.
I did not specifically learn Latin grammar by section and paragraph, and mainly developed my understanding from texts, using grammar books only for reference. The answer struck me as odd, as my internalized pattern (which often happens to be wrong because of my way of study) suggested using a gendered plural form of unum for one such thing.
An example that I immediately came accross was from (Caes. B.G., 1, 74): ut una castra iam facta ex binis viderentur: “[They mingled together so much] that two camps ostensibly made one”. A possible argument that castra could somehow be used as a normal countable noun of the feminine gender does not work here, because una facta castra (which might be considered as either
FEM-SG from morphology) does not work syntactically, as the whole NP is the subject of the verb viderentur which is plural, which makes the
*FEM-SG parse is impossible. Caesar here undoubtedly uses una with the neuter plural castra.
A&G $137.b. states [the bold highlight is mine]:
Distributives are used as follows:
b. Instead of cardinals, to express simple number, when a noun plural in form but usually singular in meaning is used in a plural sense: as, bīna castra, two camps
This conspicuously does not say anything about the singular entity. (To confuse matters further, they have to add that
(duo castra would mean two forts)
but this is not our gripe at the moment.) The PHI corpus, not being POS-tagged, does not help, as singuli is also a from of the ADJ singulum.
So what is the correct way to express one “litterae” or “castra” in Classical Latin?