This very odd one-off pistol first appeared in a 1958 Golden State Arms catalog, with no description of its history or mechanical design. I have often seen it referred to as an automatic revolver, but this is a misconception – what appears to be a cylinder is actually a rotary magazine, akin to a Ruger 10-22.
I should add that Mongo, in the YouTube comments, pointed out something I should have considered – it looks like the missing piece contained a locking block that would hold the bolt and barrel assemblies together during the recoil travel. That makes a lot of sense, I should have realized it myself. 🙂