Part No 5854-29 I think - the screw in type rather than the bayonet fitting type. As the part number implies, it came into use in 1929 and I think was used continuously from then till the end of four stroke production. Whether the various oils recommended in stampings on the cap changed over the years I honestly don't know but the vast majority look the same to me. So this should widen and ease your search.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.