This appears to be a LaserMech (or it is a 'compatible' hardware) small beam line final lens assembly.  There is a coated ZnSe 2.5" focal length lens in the assembly, it has a copper nozzle and gas purge fitting.

I listed it as 'for parts' as I don't know the condition / did not use this myself in its previous application.  There doesn't appear to be anything wrong with it.