According to Rule the 300 Win Mag was one of the few post 1959 barrels to receive an underside marking. That marking was 300M, re-designated from the 300 H&H. Also 300 Win Mag had 6 grooves compared to 30-06 4 groove barrels. I am going to say there has been widespread counterfeiting as far as 300 Win Mags. There were all supposed to be rollmarked with the the circled R in place of the "Winchester Trademark" (Winchester over Trademark) marking.
