Find out when the engine and trans were last rebuilt and by whom. It likely has two front heads rather than two front cylinders as the cylinder muffs are the same fore and aft. The heads are not. This was done quite often in the early days in order to allow for bigger porting. The rear head only has enough meat around the intake port to allow for 32 mm porting,(stock ports are 28 mm) which is plenty. Back then folks thought the bigger the ports were, the faster it would go. We now know that is not the case, (see Jim Comstock's head thread)
This two front heads mod was often done on bikes used for competition, since it is fairly extreme. I have a bike like this, it was used for sprinting in the UK in the sixties. It came to Canada in completely knackered condition and was thoroughly rebuilt by one of the best there is, John McDougall. I bought it about 500 miles and twenty years post rebuild (long story)
It it tuned to near Black Lightning levels, but my stockish 28 mm port bike which is has smallish 28 mm carbs and 8 to one cr has more power. In fact the lower tuned bike has put the run on all the hotted up Vincents around here.
So much for high performance mods! Perhaps the original designers and builders knew what they were doing?
The cosmetics of the bike in the photo look good, however it is the internal stuff that really costs $ with a Vincent. A club member recently bought a lovely looking Black Shadow that was said to be freshly rebuilt. That turned out to be untrue and the cost to do up the engine and trans exceeded $25,000.
Glen