This is a brand new protective cap to cover the rear side of Kiev C mount (PENTACON-six, Kiev-60, Kiev-88CM, ARAX) lenses. You should use it for save your lenses for dust and damages. The type of the rear caps you'll pick during the ordering process.
Kiev C mount (PENTACON-six):
Type-1. Standard cap for all ARAX lenses. 13mm deep
Type-2 (+$10). Modern cap for all ARAX lenses. 15mm deep
Type-3 (+$13). 16mm deep. Best rear cap for German Pentacon Six lenses.