When I received my donor frame, a ~1939-era Mauser 'humpback', I test-fit my 1917 'cannon' assembly on it...It 'bound' in the rear frame 'ear' area. Measurements showed about .010" of squish-in. Maybe from being dropped, or some previous owner trying to 'accurize' it, or maybe just from the heat-treat during manufacture. I didn't want to try bending 70+ year old hardened steel, and .010" isn't all that much, so I decided to use progressively finer grits of lapping compound to take it down. Starting with very coarse valve grinding compound, finishing up with auto body polishing compound. Worked excellently!
Next up was to fabricate a 'lug' for the forearm I'll be installing. This will be somewhat like a Luger carbine forearm.
(39) Sawing the lug out of steel bar, milling all sides flat and parallel.
(40) Milling top flat and parallel.
(41) Milling bottom flat and parallel.
(42) Semi-finished lug.
(43) Rube Goldberg jig/fixture to hold lug in position for drilling & tapping frame.
(44) Center drilling the lug.
(45) Drilling for #4-40 screw.
(46) Tapping the drilled hole. This will hold the lug in position while being soldered.
(47) Lug has been positioned for silver-soldering.
(48) Lug has been silver-soldered in position, using Brownell's Silvalloy 355 and Wolverine Ultra Flux. This turned out to be a mistake. This solder has too high a melting point for a procedure like this, despite Brownell's claims/ads. I should have followed my instincts/experience and gone down to my local welding shop and bought some low-melting point brazing rod. It's been 30+ years since I've done any brazing/silver-soldering and I've forgotten most of What Works. This is on now, and I just hope it sticks.
(49) Milling off the attaching/positioning screw head.
(50) Re-drilling the takedown lever spring drift hole.
Next step will be to trim the excess solder and start fitting the wood for the forearm.