Differences in magazines. These are the magazines that came originally with the guns.
From left to right:
Mark I. Follower button on the right side, chrome unmarked baseplate. Wide follower spring. The early ones had the follower-button slot only on the right side; later ones had the slot on both sides.
Mark II. Follower button moved to left side, made larger. Black plastic baseplate with inlaid silver Ruger logo with black background. Follower spring is coil type, running in a channel at the front of the magazine.
Mark III. Nub added on right front of the magazine for the 1911-style magazine release. Baseplate logo with red background.
Mark IV. Follower button has annular grooves. Notch added at the top, left front edge for the trigger "nub." Baseplate has an all-black Ruger logo.
All magazines are
backwards compatible (that is, for example, a Mark IV magazine will work in a Mark I gun, if you shift the follower button to the other side). They are not forward compatible.
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