Keith;
Apparently, the Leferver compensating screw was made in two configurations over time. I have measured two of them which had straight, non-tapered, threads. These were measured at pitch diameter, not OD, so I know the threads were untapered. What had been done was the internal thread in the frame was not tapped all the way through so the screw began to tighten as it came to position. I know that others do have tapered threads, similar to a pipe thread, though not necessarily the same taper.
You are 100% correct, the ball screw is only for taking up hinge wear & should never be tightened beyond that point to take up bolt wear. To do so will cause the barrels to not be fully closed, contacting the standing breech only at the bottom with a gap at top of the frame.
As I recall the ones I measured were around 7/16" in diameter, but don't recall the threads per inch. Pretty sure now they were a standard 60° form so could be held in a small lathe by the very end & be sized a bit with a three-cornered file, far easier than attempting to weld & fit, as you say.
Also agree 100%, if the bolting is loose make a new bolt. I have an H grade parts gun which was riding hard & put away wet as they say. The barrel is extremely pitted & has a crack, the stock is broken etc, yet it still would adjust on face & bolts tight, albeit the lever is well left of center. Amazingly with the ball screw completely removed that square shouldered Doll's Head still draws the standing breech solidly against the barrels.
In my opinion, any Lefever which has run out of adjustment has really been put through the wringer,
