In a world of infinite possibilities I would ask if anyone had your gearbox. I've been called an idiot a lot so those of you who are for some reason opposed to a person asking go ahead and use my name as a euphemism for "shit for brains". But weird coincidences do happen. What's the problem with asking?
It could have been as simple as the owner of the bike buying another tranny because they didn't have confidence or time to rebuild the one they had. The case might have ended up anywhere after being rebuilt. It might have been exchanged with the person who built the existing tranny. In a small world, its possible someone has it.
Personally I would enjoy a little more brotherhood here and a lot less antagonism.
Have a good day
Oh and welcome to the forum. There's lots of friendly people here. Maybe a silent majority, but we are here.
Russ