Used Canon 50mm Lens Cap 

ID is 50mm. OD is 53mm. It is too large for 55mm and too small for 49mm. I think it should fit Canon FL Lenses with 48mm rim. Since it is a slip on type cap.

 

Cap has been used, but it is not damaged in any way, no cracks or dents. This lens cap is fully operational and functions as intended.  I would rate it an 8, on a scale from 1 to 10.