2½” Chrome wing nut cap, hinged. As fitted to :-
Triumph 3T,5T,6T,T90/T100 and TR5 Trophy models (1938-50).
Norton 16H,ES2 (1936-54)
AJS/Matchless competition models (1954-65)
Can also be used on BSA DB34,DBD34 Goldstar models to allow venting