This is a representation of a Ruger Standard model with the 4-3/4 inch barrel. These were also available with a 6-inch barrel. Magazines hold 9-rounds. The safety acts as the bolt hold open when manually engaged.
Here is a version of the Mark I Target with 5-1/2 inch bull barrel. Other versions have the 5-1/4 and 6-7/8 inch heavy tapered barrels. Target models have adjustable rear sights.
Mark II pistols have some very important changes in design over the Standard and Mark I pistols. The magazine now holds 10-rounds. These pistols have a bolt hold open device once the magazine has been emptied and relief cuts at the re4ar of the receiver for a more positive purchase on the bolt ears. Many more barrel variations are available on the Mark II's.
With the introduction of the Mark III style in 2004, we see one excellant addition in the magazine release position. Other additions are a "Loaded Round Indicator", magazine disconnect system and a locking mechanism for the mainspring housing. Magazines are still at 10-rounds and with the magazine disconnect parts in place the magazine must also be installed to fire these pistols.
With the introduction of the Mark II 22/45 Ruger began to use a "kydex" grip frame that has a grip angle very similar to the 1911 style pistol. These pistols were the first models to employ the 1911 style magazine release.
The most recent Mark III 22/45 RP models, including the "Lites" have grip panels that are removeable, a Loaded Chamber Indicator and the magazine disconnect parts.