[Mark]

The low serial numbers you mention were carburized using a "pack hardening" method that was acceptable in that time. The issue that came about was the carbon content in the steel allowed for a "through hardening" that caused the steel to become brittle.
The steel wasn't like glass, it required repeated firings to work harden itself. It was then that the steel came apart.

Receivers were "caught" at the 800K mark, annealed, and re-heat treated, then drawn back. These are good to go.

As far as firing the low numbered receivers, how lucky do you feel?
Mark