I was shown a 1913 Colt 1911 with a same period 22LR conversion unit. On the bottom of the frame dust cover, two drilled/tapped set screws were placed into the dust cover symetrically opposite each other where the frame rails meet the slide. They may have at one time mated into the rails but unsure. Anyone with info on this?

Back in the day, some pistolsmiths used that method to tightened up slide to frame fit, to enhance accuracy.

Thanks for the replies. I had a chance to see the gun again today and the set screws actually bear against the barrel bushing. I was able to take the gun down to really take a good look and was amazed my this idea. Must not have been much bearing surface as the bushing had no problem in turning. I should have said the set screw were located on the slide not frame just back about 1/8" from the end.