I can only go so far with what I can contribute here, but when the MKII came out, lots of folks thought that anything which preceded the MKII was a MKI. The MKI was the target model only (in several varialtions) and the fixed sight models were the "Standard Auto." The MKII came along with a bolt stop to hold the bolt open after the last round was fired, a ten-round mag. scallops at the back of the reciever to facilitate a better grip on the bolt ears and tapered bolt ears, which were phased out fairly quickly. There may be more to the MKII which I have missed here but that's the basics from its "early days" and that's from memory. The MKIII, of which I know little, added a loaded chamber indicator and possibly other features. The MKII was the first to be offered in stainless steel except for a run of 500 commemorative Standard Autos (with "red eagle" grips) near the end of their production in the early eighties.
My own Ruger Auto (down to one) is an early seventies MKI with a 5" bull barrel, Walnut grip panels with the thumb-rest and Micro sight. It looks nearly new for all the shooting it has done and is one of the most accurate handguns I have ever owned with the cheapest bulk Federal ammo available locally. I have owned a number of Standard Autos and MKIIs but never a MKIII. All have been reliabel and accurate for me but I love that old MKI.
Others will certainly have more to contribute and I hope they will correct any errors or omissions on my part. I am not a collector and am not a Ruger "expert."
Jeff H
