Latin mecum, tecum, etc.

[petegray]

> The same question comes here
> with Attic "peri": Why did it keep the possibility of
> being a postposition while no other adpositions did?

Any two-syllable preposition can occur after its noun, except ana, dia, and
amphi.  The position is highly marked, and the occurrence rare, but examples
do occur.    Sometimes the preposition is used adverbially; sometimes it is
a case of tmesis, sometimes a genuine post-position.  In all these cases,
the accent appears on the initial syllable.

You are right in this respect, that in prose the only examples we have are
with peri.

Peter