I've only ever seen the word 'pyx' used in the context of a container for the Eucharist.

.pyx is used by Cython https://cython.readthedocs.io/en/latest/src/quickstart/cytho...

It can also be a coin chest.

I think I saw it used as part of an answer for a H. E. Dudeney puzzle.

Ah, yes. A Google search came up with it being the answer to problem 51 in his "The Canterbury Puzzles" on page 202. The link is https://books.google.com/books?id=ce5LDwAAQBAJ&pg=PA202&lpg=...