Back in the days. An 500 cc engine used to be a tough boy. A machine that was usually not only driven because there was no money for a car.
BSA's B33 was such a fatter. The single-cylinder 500 cc and a proudly specified capacity of no less than 23 pk!
The B33s were solid engines and quite a few were sold between 1947-1960. At the time, they did evolve from clearly based on 'pre-war' to modern with a telescope at the front and a swing arm at the back.
Those B33s looked like some civilian B34 Goldstars, the sportsmen
BSA house. And that has caused many surviving B33s to make a genetic switch. Because without much effort, such a good B33 could look - roughly - like such a fantastic B34 Goldstar.
That was also done enthusiastically. And as the prices of the rare Goldstars increased, the boosted B33s evolved into counterfeit B 34 Goldstars.
If you can adopt an authentic B33, then you are lucky. When considering purchasing a B34 Goldstar, it makes sense to engage a brand specialist or the BSA owners club ...