The original M1905 bayonet for the M1903 rifle had a blade of about 16", this was the bayonet used in WW1 and up to almost WW2. As originally produced they had wood hand grips. Shortly before WW2 the wood had grips were replaced with black plastic (like Bakelite), and the original metal/leather sheath was replaced by an OD plastic one.. These M1905 bayonets were common in the early days of WW2, and in units in training. Reportedly some Engineer units liked the long bayonets to use as mine probes. As W2 approached and it was apparent that the coming war would involve more trucks, airplanes and other mechanization. The US Army (and others) found that the long bayonets would be unweildly, so the blade length was reduced to about 10". These bayonets were designated as M1 bayonets. The Army had a huge stockpile of M1905 bayonets, so many were ground down to the new length. I have seen these referred to as M1905E1 or "M1943" bayonets. Some units used the long bayonet in combat, particularly early in the Pacific campaigns. There is an often-reproduced photo that shows infantry supporting a Sherman with a rifleman in the standing position with that great long bayonet fixed.
Anyway, on its blade, just near the hilt, your bayonet shound have an Ordnance "flaming bomb" insignia, plus a date and two to four letters to designate the maker.
A really GREAT reference is Bruce Canfield's book "US Infantry Weapons of World War 2". Note cheap, but it covers every type of small arm, machine gun, grenade, flamethrower adopted for use by the US Army in WW2, It covers history, makers, collector notes, etc.