The 800,000 s/n was the cuttoff for rifles marked " Springfield Armory" on the receiver ring. Those below this number were produced with single heat treated carbon steel, the so-called "low numbers". These low number receivers were considered brittle, and prone to failure, and were withdrawn from service by the US Army as a safety precaution. For rifles produced by Rock Island Arsenal the low/high number distinction occured at s/n 285,507.
All receivers made by Remington and Smith-Corona were made with modern high grade alloy steels. Conventional wisdom is to avoid shooting low numbered receivers and most owners choose to do so.
National Match M1903s had the bolts numbered to the receiver. Others did not.