It was the steel they used. I think the Chec Mausers were the only WWII rifles with a better grade steel. Some of you Mauser types might know if it was the Chec's or the Polish Mausers that had special tool steel receivers, I am not sure anymore which it was. Other than that, the steel and heat treating process seems about the same for Mauser or Russian receivers. Nothing wrong with it. The case hardening makes the Mauser and Russian more resistant to abrasive wear and does make the action smoother.