Got the receiver completely disassembled. Only one tiny screw fought me. Little soak while I took apart the rest, and a good scrub of rusty gunk out of the slot and it came right out. Was much easier than I thought. Then again, there was a baggie with what looks like the sears, associated levers etc. in the box. I took pictures as I went, and put away the left and right parts separately, just in case they are slightly different as can be on other similar models.
I haven't looked in the baggie with the sears yet, but all seemed to be on the receiver except one small spring...
There is supposed to be a spring under this screw.

Well, there was one. I thought something was fishy - even before I took the main spring out (under that slotted bushing), there was loose floppy slack in the lever before the tension of the mainspring engaged. My guess is that spring is tensioned by the screw, and has something to do with adjusting when/how it locks up. If I can't find a parts diagram, etc., I suppose I can get a cheap spring kit and just wing it.

I think now my plan is to conserve the receiver and all its parts, then assemble everything to make sure I know how it works and confirm everything is there in working order. Passing that check, I may just have to do a detailed disassembly instruction, video or written, with all the parts listed, since no such thing exists on the internet in complete form (did find a guy that did some videos restoring one, but they are far from thorough).