Tank top rubber plug. 1.437" diameter spigot for inside tank. Used on many BSA models with fiberglass tank - Hornet/Shooting Star/Star Fire/Firebird Scrambler/Spitfire
This cap will fit any motorcycle with a 58mm fill hole
OD is 85mm
This cap was common on German motorcycles including BMW /2 and some earlier tanks, Zundapp and Puch.