Backstrap plugs fit into the bottom of the backstrap and pin in place. The Flat Plug closes opening in bottom of backstrap to create a smooth, cosmetically appealing appearance. For use when you are not attaching the magazine well.
The Magwell Plug is for attaching the polymer magazine well. The plug is designed to yield and break if excessive force is applied to your magazine well to prevent damage to the frame. Get a few extras in case you need them.
These are meant to fit only Shadow Systems Proprietary framed pistols, the MR918 and MR920.