Also, most of these "two groove" M1icon barrels were made-up during the 1970's when the scrap demil M1 rifle barrel stubs were available and two groove 1903A3 barrels were not expensive to buy. Some people were also using the demil receiver halfs to assemble a receiver too, Some of these people did excellent work while others did a poor job. It is very possible to find a M1 rifle with both the receiver and barrel (two groove) put together from demil parts. Once in awhile you just might find a four groove 1903 barrel that was used in a scrap M1 barrel stub too.
The two groove was tested for the M1 during the war and I understand was satisfactory but was never used, you might find a Marlin barrel someday that has more than four grooves too