I'm no expert, but the number on that receiver is very close to the change to double heat treatment. From "The 03 Springfields" by Clark Campbell:
"The exact number of the receiver at which Springfield Armory switched to the double heat treatment is unknown." And a bit further on in the same paragraph:
"Receiver No. 800,000 was completed on 20 Feb. 1918, and it is known that all receivers after 800,000 received the double heat treatment. It would seem, however, that even the number 800,000 is open to question in view of the uncertainty of the date and the fact that over 1000 receivers a day were being made. Thus the fact that receiver #801,540,which was broken in 1929 when fired with a 7.92 Mauser cartridge, is the only receiver recorded as having the double heat treatment to have been officially reported as broken..."
I'm not saying, I'm just saying.