In 1949 William B. Ruger started his company with his partner Alexander McCormick Sturm, and the gun they designed was the .22 cal target pistol due to the fact that they wanted to produce a gun for recreational purposes.
They produced that pistol and refined it's design with approximately 30 production modifications. When they introduced the Target Model, they named it MK I and the original pistol remained as the Standard .22 Pistol to differentiate it from the Target Model, these pistols were produced until 1981 when 32 yrs of production had improvements in design and production techniques that yielded the MK II.
The MK II had a few improvements that included:
1) Increased Mag capacity from 9 to 10 rnds
2) Automatic bolt stop when the last rnd was fired
3) Flats Milled into the rear of the receiver to make the bolt easier to grab.
4) Enhanced bolt grab tabs
5) Improved trigger design
6) New safety mechanism
7) A slide release
8) Available in Stainless Steel
When the MK II came into production, all previous versions (Standard and Target Models) became officially the MK I
Along with the MK II came for a brief time in 1992-1993, the .22/45 that sported a 1911 government model grip and mag release design.
In 2005, MK III was introduced, some changes included:
1) Frame mounted mag release (adopted from the .22/45)
2) Loaded chamber indicator
3) New safety design
4) Re-contoured ejection port
5) New bbls designs (fluted, slab sided, ect.)
The MK III represents 62 yrs of true and tested performance of a gun that has changed little from it's original design.
1) The .22 cal Pistol was purposely made with a resemblance with the German Luger
2) The fundamental inspiration for the MK I was the Colt's Woodsman and the Hi Standard .22
3) The first advertising was made in 1949 in the august issue of American Rifleman
Combat Handguns March 2012 issue