I often wonder how things like that come apart too. Maybe they got taken apart to be cleaned or repaired by someone who for whatever reason never got back to the project. Another pet theory I have when I see this sort of thing is that there was something else wrong with the gun and the good part was taken off to be used on another gun, so the whole basket case I'm looking at may be crap. Never know until you really dig into a project what you've got. I've picked up a few that were bags of snakes, and I have a couple of prized shooters that came to me in a shoebox. That's what keeps me messing with them. Sometimes I save one and it will go on for another generation, cause guns can and will last longer than one life time.
I knew an excentric guy who did that to bunches of motorcycles (taking them apart I mean, not fixing them for future generations). I saw him on a rare flat head Triumph one day. He stopped and told me all about it. Complete bike with all the RAF boxes and stuff on it. Made for around the airfield transport for the Brit military. Original paint, running and driving, all that. A couple weeks later I dropped by his shop, he'd taken it apart for reasons unknown. When I helped him move a few years later. I found parts of the bike in three outbuildings and the boxes under an empty schoolbus. Real shame, that.