I wish I knew, honestly. I bet that it is "missing" from somewhere. Thats why I was so surprised to see it just laying around in the attic. It might have been damaged or missing pins or something, when I picked it up, the upper receiver basically just about came out. Hinged forward from the stock/lower and gave me a good look at the internals. I just kinda put it back down where I found it lol. Wasn't expecting it to be as heavy as it was.