The butterfly oil cap I think was first used on BB Goldies. I bought a bike minus engine in the 70s but complete otherwise and the frame is one the first swinging arm Goldies so is a BB32A, 1953 registered. You could check which frame your has since might be an early one, had to be re-registered in 71.
They may also have been used on the BB32R frames since they had them on the Daytona Goldies in 54 and may have been used on the trials type bikes that those frames were made for.
I have a parts manual
for it, shows clearly the oil cap. Frame is markedly different too, solid lump of steel brazed in for the oil pump kink and quite narrow under the gearbox compared to CB frames, plus a few other small mods.