What Did the Romans Ever do for Us? IV Mystery Cache

Some Roman currency for you:

Sestertius ≡ Denarius, Quadrans ≡ Dupondius,

Uncia ≡ Semis, Semis ≡ 2 Dupondius, Uncia ≡ Triens,

Quadrans ≡ Semis

As ≡ Dupondius, Quadrans ≡ 2 As, Sertertius ≡ 2 Denarius.

This is the fourth of my 'What Did the Romans Ever do for Us?' caches. Hopefully this shouldn't be too hard with a bit of lateral thinking - the puzzle part is harder than number 3, and finding the cache itself should not be too hard. Cache is a small micro.

Puzzle rating = 3.5, cache difficulty rating = 2.5

You can validate your puzzle solution with certitude.