on Preunit BSAs, I cannot say with 100% authority, But it would seem that there was a lot of parts bin engineering as sorting out project bikes and basket cases has required a lot of detective work trying to sort out what fits what, as well as "Was this factory?"
I spent a LOT of time earlier this year trying to sort out forks and triple trees and BSA alone consumed a lot of time and I still have questions.
Aside from deciphering the fork legs and variants, Top trees has been challenging. In unit twins, there was 3 variations and then a final version meant as a univesal replacement that had me baffled for many days until I came across a clue that resolved it. However in the mid 60s they use a drooping mustache top tree, But then reverted back to the preunit type.
Not everything has a casting number and part number, But in the early type, Preunit you have to look at width of the tree, Then look for instrument bracket bosses, locks and on the lower tree the steering stop cast features.
Some competition models dont have ANY bosses or holes, Some bikes HAVE bosses cast in but not machined, Some of the instrument bosses protrude quite high and others are flush or nearly so. A nacelle model will in theory differ, as it doesnt use the instrument brackets, But I have a couple top trees that are baffling. I also found it helps to look at as many other examples as possible, online and in person, Find someone with a known history bike and get close up pictures. Frequently, The parts books are just a rough guide.
Rivet counters can go mad. If you find a cross section of original and known history machines that are consistent, go with that, Period advertising is probably the WORST examples to use, Some are airbrushed, Some were mocked up prototypes for photos.