Used Asahi OPT. CO. Plastic Rear Lens Cap - Japan 6222056
Not sure what lens it might fit. It is too small for PK lenses, and too small for M42 lenses, inner diameter is 39.7mm. And it is 7.5mm deep.
Item has been used, with little signs of cosmetic use from normal use. But it is not damaged in any way, no cracks or dents (see pics). This Cap is fully operational and functions as intended. I would rate it a 9, on a scale from 1 to 10.