Seats
Robert: I have a 1921 Sunbeam. It doesn't have a Sunbeam seat. The bike is completely original and unrestored. Savvy owners specified (I'm making this up because i can't remember the name) "Acme seats, the mark of the experienced motor cyclist". Maybe it was Excel. If it wasn't raining and dark, and the yard full of dragons, I'd go and look.
The point however is that before the war (WW II), manufacturers stuck on SOME accessories that were cheap as chips, because they knew the owners would throw them away and substitute whatever was fashionable to avoid being laughed at by their friends. Terrible thing, peer pressure: look at all those poor sods turning the clock back and substituting cadmium plated mild steel nuts for the stainless nuts PEI would have used if cost had not been an issue.
Dunlop rubber saddles, in my experience (which is not comprehensive), were generally factory fitted only to trials bikes. All others had metal pans, leather covered. I'm not entirely sure that Dunlop rubber saddles weren't in fact a cheap alternative to "real" saddles.
Tom