As well as locating parts on fleabay, period photos occasionally turn up.
This one has "July 1932" handwritten on the reverse, along with someone's initials - no doubt the dapper gent in the photo. Unfortunately I've no idea who the gent is, or where it is taken. Detail isn't too clear - it is definitely ohv, it has the smaller diameter brakes, so it is either a 350 ohv or a 1932 season 500 (with Burman gearbox concealed beneath the covers). After another look, there is an external oil drain tube at the front of the engine, returning oil from the rocker boxes to the crankcase oil tank, so it is a 1932 500 ohv.
Wonder if he turned the flat cap round whilst riding, like all the boy racers of the time?