Hi Alan,
I happen to own an OK B-31 which is the years before's model but reg 1932, its frame number is 162**
The 31 model has Webb girders not the pressed steel ones fitted to your bike
Mine has the 285cc JAP engine and 3 speed Albion box, it goes very well for a small sidevalver,
On mine the saddle sits on a 7/8dia piece extending from the frame lug, not the bicycle type design on yours
You have a major challenge on your hands
I think the petrol tank will be the most difficult thing to find??? But heres hoping
I have been having "bad thoughts" about mine and have sourced a 250ohv JAP engine (which happened to turn out to be from an OK
) and a 4 speed gearbox, Of course both need rebuilding so its a while away yet!!!
Best Regards
John O R