...was this little bit of metal:
This is the lower part of the Rear Filler; it sits between the magazine well and the trigger group and keeps the latter from falling out of the gun. That is, the complete part does. I didn't want to take apart the rear of the magazine well and rebuild all that from scratch, so I just milled out a space:
I was using the wrong end mill and pretty much getting everything else wrong (notice the groove in the side rails from an incautious pass after something slipped on me!) Which explains why getting this far was eight solid hours of machining.
Well, I did re-mill and clean up the slot where the sight sits. I haven't made up my mind whether I'll try tap holes for screws, or just press-fit brass pins which I can then hammer out as rivets.
Sigh. I'm trying to avoid cutting too close to the magazine well because the rear slot would be a real pain to clean up. But based on what it all looks like now, I think I need to mill those side rails in a little further to properly expose the slot where the new part goes. And, yes, I am tempted to cut the entire old part away, but this is all hardened steel and it would be very hard to align it in bandsaw or anything else more suited to deep cuts than the manual mill.
Yes; at this point I've learned enough about how the weapon goes together, and how to derive the various dimensions of the parts, that I could now start from raw materials and fashion one completely from scratch. In aluminium, that would almost certainly be faster than all the work I've done so far re-welding and trying to clean up.
(Or even steel -- especially since you can get parts and parts kits that include, intact, the magazine holder, trunnion, and even lugs -- leaving pretty much the rear part of the tube and the side rails to fashion from scratch).