Good info above, and I may add we had heard many years ago that one of the main reasons for the change to the A-100 frame was in preparation for the addition of a slide (bolt) stop, ala MK II.....if you look there was a 'hump' (raised portion) on the left side of the A-54 to allow the magazine follower 'button' to ride up,into the frame.......they moved this 'hump' over to the right side, thus the location of the bottom frame /grip screws also changed...they "needed" the flat to be on the left side, so the shooters hand could access the lever. This also called for a double slotted magazine so they could MOVE the button to the right side also, thus the early change to the A-100 and the newer ,two slotted magazine...and yes, allowing folks to swap the button on the newer A-100 style, and go back and use it in the A-54....design and engineering changes obviously, and good marketing to be able to continue to use both.....sadly the this reverse changed the grip panels to NOT be able to use one, or the other...though later on the Mk II would work in the later MkI ( and standard autos)
and it did take them almost 20 years later to come out with the Mark II's, so I would agree as to making the change at the time of having to come up with NEW stamping dies.
the bottom of the early magazines is actually chrome plated,not stainless or nickel, getting harder to find the early mags these days, but the MK IIs work, as well as hold an extra round.............
what adds to much confusion is how many 'frames', as well as uppers, have been swapped around and placed on other guns , to make them work or build them up from scratch........so one has to check, look see, just what markings are what, and where the holes location is for the grip panels...and know the difference from a standard auto, Mark I and then the Mark II....... :roll:
One of the reasons we prefer and like the Hiddleson book 'Encyclopedia Of Ruger Semi automatic Rimfire Pistols-1949-1992" :wink: