Thanks for all the help guys.. This question wasn't for a build, well, not right now. This was research for the purchase of a secondhand gun. The gun in question was built using 2 11/32" rivets, with the balance being buttonhead screws. The receiver is welded under the bottom plate to both side plates with drilled holes and then welded back shut. That would be permanent as hell, so I think compliance has been taken care of. That was my major worry.
I have lurked, learned and posted on here for a loooong time, as many of you know. I value the advice given here, which is why I asked after I had already read all I could find.
I was leery of a screw-built and welded 1919, but after doing some more reading, this hybrid screw/rivet/weld version seems to me to be quite solid, especially as the major rivets are installed. After reading the many postings on this, especially OhioBlackSheep, this looks like a decent way to go. Not the only way, but decent. I'm going to do a hands-on inspection of the gun this weekend. If it's as good as the owner claims, we'll both be very happy, and I'll have another way to burn through ammo.
I see a possible Sokolov mount refurbishment project coming up
( I hate lugging crap from the truck )