In a last hopeful attempt to get a rifle adopted and produced for the US military, John Pedersen designed his own copy of the M1 Garand rifle in the late 1930s (approximately 1939). His toggle-locked rifles had been irreversibly rejected, and the Garand rifle fully adopted by 1936. Pedersen's exact reasons for making a copy of it are not recorded anywhere I can find, but he did make a number of small changes to the design.
Two series of Pedersen Garand rifles were made, first the GX and second the GY. Only 10-12 of each were ever made, so they are extraordinarily rare today. They all show some detail differences form the M1 in stock design, sight design, etc. However, a persistent question had long been, what differentiated the GX from the GY? When I had the opportunity to examine one of each type side by side, I knew I would have a chance to determine that answer.
The meaningful differences are twofold: clips and gas system. The later GY rifle uses a standard M1 Garand clip, while the earlier GX rifle uses a distinct Pedersen-designed clip. The gas system difference mirrors a change that the Garand underwent as well. The early GX rifle was originally made as a gas trap style of action, in which the rifled barrel ends just shy of the muzzle, leaving an unrifled and minutely larger bore with a large hole to capture gas pressure just before the bullet exits. This was believed to prevent problems with wear and accuracy caused by drilling a gas port in the barrel itself (the German military also believed this to be a problem, as one can see from their Gewehr 41 development requirements). However, this system instead led to reliability problems, and by 1940 the M1 Garand had been converted to use a normal gas port. The later GY Pedersen had the same change to its manufacture, and the early gas trap GX in this video was retrofitted to that system.