BSA made their own tanks
From what I have been able to work out , the post war single piece top tanks had only 2 press dies
To get the different size & shape tanks the top was ( or could have been ) just trimmed differently from the pressing.
I was looking at making tanks a long while ago and when looking at making masters noticed that most were actually the same .
This is particularly true for the single bolt tanks where just about all of the tanks can be made from a single top pressing .
What does vary is the bottoms
there is better than 20 of them which makes perfect sense as they are shallow pressings so the press dies are about 1/4 the price of the deep dies used for the top.
And they can be pressed from cheaper steel in a much smaller press.
This is one of the reasons why the unit engine bikes used 3 piece tanks which would have nearly halved the price of the 2 piece tanks .