The previous owner admitted tapping the take down pin from right to left with a nylon mallet resulting in the break in finish on the left interior of the frame hole. The upper is tight to the frame when the grooved take down pin is in its locked position.
The "reinforced" (stamped and welded plate) isn't a reinforcement. It was done as a cost cutting measure after the move from Powder Springs.
The stepped barrel, take down lever, the U notched lower to allow access to the take down lever, magazine well geometry, feed ramp geometry, adjustable front sight, sear geometry, stock block geometry, two piece strap hanger (welded) along with the multi-part welded shoulder stock , coil spring under the trigger... are just a few of the unique nuances only found on the original Powder Springs M11's.
MAC collectors and aficionados will appreciate the thought, time, and effort put into the original guns which makes them unique and desirable to some individuals. The average shooter who simply wants a gun for phone booth gunfights is better suited with the later iteration weapons.