There are two types of locking fuel caps:
Mk 1 - Fits Talbot Express, Fiat Ducato, Citroen C25 and Peugeot J5 petrol and diesel on vehicles up to December 1990. This features a screw on system including two keys.
Mk 2 - Fits Talbot Express, Fiat Ducato, Citroen C25 and Peugeot J5 petrol and diesel on vehicles after January 1991. This features a flush fitting bayonet system including two keys.
A-Class - This is the most commonly used locking fuel cap found on A-Class built campers, it feature 3 lugs on the under side rim of the cap. This comes in white and black as white is for water tanks and black is typically for diesel.
*These items does not fit A-Class campers*