[Gil Martin]

gumpokc,

The answer to your question is best explained by Smith and Smith. I have quoted from their classic reference work, "The Book of Rifles".

According to Smith and Smith in "The Book of Rifles", Springfield receivers below 800,000 "...were made of Springfield Armory class C steel. These are commonly considered 'brittle' receivers. They should not be used with modern or stepped-up loads. Receivers between 800,000 and 1,275,767 serial numbers are Springfield Armory class C steel which were specially heat treated by improved methods. These are not as desirable as the later numbers, though far superior to the earliest manufacture. All receivers between serial number 1,275,767 to 3,000,000 are made of nickel steel WD 2340 and represent the most advanced type of receiver designed..." The authors further state ..."Springfield receivers made at Rock Island Arsenal between serial No. 1 to Serial No. 285,507 were made of Springfield Armory class C steel. From No. 285,507 to 519,921 were made of Springfield Armory class C steel with improved treatment. Receivers over serial No. 319,921 were made of both class C steel and hot rolled nickel steel WD 35NS..."

All the best...
Gil