very much a bespoke build; I note the norton commando footrest and conical rear wheel.
Nicely done and the BSA frame is certainly solid. I'd seen this bike on the vincent owners site too. People put vincents in all sorts of frames, commonly norvins but that usually involves cutting off lugs from the engine.
A vincent doesnt need a frame, it normally has an oil tank bolted to the top which has the front steering head bearings in, and a rear swing arm; both of which are easily removed.
The engine alone is worth far more than a genuine RGS.
There's an equivelent harley engined BSA twin special in NZ that a mate of mine built up from a bare frame. each to their own.