OK, found it.
This is from a USMC armorer.
Replace any spring that:
Is less than 19" free length regardless of round count.
Is kinked or does not lay straight when out of the gun.
Has flat spots on the coils.
Has >2500 rounds on it or unknown rounds in the case of a M1you bought.
Do not use "extra power" springs or a spring longer than 20.5" long.
And do not use ammo that is not USGI spec or equivalent handload. Most commercial 30-06 ammo is loaded with powders that are not compatible with the M1. Federal and Hornady load ammo that is specifically designed for the M1 and is so labeled. Prvi Partisan also has a M2 Ball equivalent load.