"When were the serial numbers applied to the receivers?"
We have had discussions on this before, except for a few proof marks applied with a single letter punch I think the serial number, model, and Sringfield Armory were done before heat treat. The larger more difficult to make stamps would have lasted longer on the soft steel before heat treatment. I would guess the numbers of some of the receivers scrapped during and post heat treat would have been replaced. With an occasionally duplication.
After too long after any scrapped numbers may not have been replaced. Maybe that is why some serial numbers are higher than the total produced?
I was not there and this is only guess work on my part and I have been wrong many times.
We saw the case hardening boxes on the floor in recent photos. Was more than one receiver done to a box? I can not see how they would be done in order, with no number on the box. I would guess cleaning and inspection was done after unpacking with no attention given to the numbers unless a scrap one was found.