Your premise seems to be that somehow, and for whatever reason, owners have removed perfectly good gearboxes, engines, or indeed even frames, and fitted other such items that assumably were made available due to similar actions by someone else. I find that ridiculous. The reason that your bike, for instance, has a non matching number gearbox is almost certainly that the original shattered to pieces at some point, or at least suffered irreparable damage of some kind, and a replacement used gearbox was found from a bike that likely was wrapped around a tree. When you see engines and gearboxes in non matching frames, it is almost completely certain that whatever bikes started life with those components have long since bit the dust. There could be other reasons in a small number of cases, like perhaps a 5 speed upgrade, but that would by default preclude the owner ever wanting back something that he sold because he no longer wanted it in the first place.