I dont know how many here remember the SMG "parts kit" boom of the 90's, but it was a brisk business, and there arent that many taxed guns in the registry requiring that many spare parts. They sold a ton of kits, and for years.
You also used to see all sorts of "tubes" at gun shows and in SGN at the time as well. Some tubes even came marked with a print, so all that was needed, was something as simple as a dremel and some hand tools, to get them ready for assembly.
Hmmm. What then, do you suppose happened to all those kits?