Just lucked into one of these. The slide was made by Peters Stahl, who made the Springfield Omega. This is the rarest of the 22lr conversions for the 1911, and certainly the most accurate. Uses Colt Ace magazines. Surprisingly, it uses a light 1911 recoil spring in a reverse plug configuration. Shims are used to fit the unit to the gun. I decided I'm going to fit it to my Omega frame, turning the Omega into a dual caliber Peters/Stahl gun.