Score!

So, in addition to the single-carburetter A50, BSA also made a twin-carb version of the 500cc for competition. It was called the Wasp (but there was also the Cyclone - little difference I think) both in scrambler and in road going trim; not many were made compared with the standard version so it is somewhat of a rarity compared to the twin-carb 650s.

Really, the most important component of this version is the cylinder head, which has the twin port for two carburetters. Visually the biggest difference is in the space between the two combustion chambers, about two inches on the 500, whereas there's hardly any meat in between the 650's head.
A few months ago I was browsing around for spares when I came across a spares dealer (NOS and new) that had one up for sale at a very reasonable price and in good nick. I jumped at the chance and now I have the option to transform the engine into a fire-breathing twin-carburetter beast if I should find that the standard head doesn't have enough go.
What the heck, I just like having it sit on my shelf for now!