Introduced in 1918, the Model 51 was developed from patents and designs from John D. Pederson of Pederson Device and Irwin-Pederson fame as a hesitation-locked blowback semi-automatic pistol with multiple overlapping safety features and good ergonomics. This particular example has been extensively cut in multiple locations to show the inner workings. An emphasis is placed on the lockwork inside the grip frame and the locking breech block in the slide, the former receiving a large window on the right side and the latter exposed on both sides. A brass plaque atop the slide bears the serial number, with the two-line address and patent marking (ending in "PEDERSEN'S PATENTS PENDING") on top and a pair of checkered and riveted Remington-UMC marked grips, which have been cut to match the openings in the frame.