The M1 Garand was adopted on January 6th, 1936 by the US Army. Unfortunately, the M1 Garand experienced an unjustified level of jamming early on in production and, in 1939, the gun underwent a redesign to remedy the issue. Almost all of the original M1 Garand rifles were rebuilt to accomodate the redesign, and an additional estimated 4 million rifles was produced during WWII. Other than the sniper variations of the M1 Garand, which had a sniper's scope attached little mdification was actually done to the rifle in 1945, after it's initial redesign.
M1 Garand on Wikipedia