Hi, Defender:
Mike is right. It's the whole receiver that's brittle on the low number Springfields. See Hatcher's Notebook, Chapter IX, Receiver Steels and Heat Treatment. Rock Island numbers after 285,507 and Springfield Armory numbers after 800,000 are OK, with a few exceptions. General Hatcher notes that a few Springfield Amoury receivers got set aside and were later issued with 800,000+ numbers. #801,548 blew up.
The military load was a 150 grain bullet at 2700 fps, compared to the current commercial load at 2900 fps.
Bye
Jack