This flush fitting cap is slotted for opening with a (separate) slot style or 2-prong key.
It features an O-ring seal, and a braded stainless steel keeper chain.

This umarked cap fits the following Attwood deck fills, only:
66406 - Fill body marked Gasoline
66407 - Fill body marked Diesel
66408 - Fill body marked Water
66408C1 - Fill body marked Water, in blue