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Throughout the first 33 years of continuous production, the basic mechanical operation of the Standard Model and Mark I pistols remained essentially unchanged, just as Bill Ruger had originally designed it. The differences among them consisted only of variations in barrel shape and length and type of sights—and one small modification to the grip frame was made in 1971 when the original forming dies for the two halves (after 22 years) finally wore out. The new dies were made so that the cut on the bottom of the grip frame where the magazine follower button fits was on the left side, which is the opposite side from before. The new frame style was designated the “A 100” frame and was stamped as such under the left grip panel. Pistols with A 100 frames are termed different from the older guns in that their grip panels are not interchangeable (the Ruger eagle medallion is on the right panel on the New Model guns), and older magazines cannot be used in these later guns. The magazines made for the newer guns, however, have button slots on both sides and can be used in older guns simply by switching the button from the left to the right side (still an important fact to know, given the hundreds of thousands of older guns still in operation and circulation).
Notice the red circles on the below images, it shows the "high" screws location change in 1971.
I believe the post '71 pistols are compatible with Mark II grips as well. I will verify this over the weekend and report here.
Notice the red circles on the below images, it shows the "high" screws location change in 1971.

I believe the post '71 pistols are compatible with Mark II grips as well. I will verify this over the weekend and report here.