Used Vintage Caltar Front Lens Cap Φ42 2103037
It is slip over type cap. I tested it on 40.5mm filter rim and it feels little bit loose. But it seems to be made for 40.5mm lenses. I don't think it would fit 42mm.
Item has been used with signs of cosmetic use - it may have some scratching from normal use. But it is not damaged in any way, no cracks (see pics). This lens cap is fully operational and functions as intended. I would rate it a 7, on a scale from 1 to 10.