This should be a new post. One of the big reasons I like my old Mopars. There are so many manufacturers and models out there, under criss-crossed badging, that parts management would be impossible without computers. Even then, one bad entry in the master database and an available part just vanishes. I suspect that might be why I can't find rear 10" drums for my 65 Dart, despite false leads.
In the 60's, it was pretty simple, mostly either GM, Ford, or Chrysler parts, each in 3 body sizes, with many parts interchanging. Even thru the 70's, Jap or European car parts and service was expensive and specialized. Even earlier, you could buy a suspension part for a Model T at every country store. Woe will be those who try to keep current cars running 40 years from today.