Yep, could well be a racing frame built to house a Vincent engine. A lot of folks don't know that back in the day, you could just buy a Vincent engine from your local dealer, and a lot ended up in all manner of race bikes, predominantly speedway outfits. That's why there are more Vincent engines out there, than there are frames.
Did the swingarm pivot from the back of the gearbox on a Vincent? I think so, and I've seen a vincent with the tank off, and that large frame backbone looks familiar, but then again, it also looks similar to some of Fritz Egli's Egli Vincents, but they all had disc front brakes, so maybe not. Very interesting though, and a quick sandblast and coat of paint would fix the "dragged out of a drain" comments.
