Sure wish someone had asked all these questions before all the guys doing the work had died.
When hand stamping stuff you sometimes do not hold the stamp square and get a very heavy impression on one side and not anything on the other. You try to fix it by setting the letter punch in the deep side and rocking it over then striking it again. About half the time it works real good. The other half not so much.
But on these long words like Springfield Amory, they would have had a single punch with all the letters or a frame with replaceable elements, probably applied with a lever press or the like. Today they would have a roller die that would travel the length applying one letter at a time. But not sure if they had that capability back then or not.
Several possible causes for that one that was deeper on the ends, the receiver could be low in the middle or if misapplied on the first impression lined up by hand with too much correction on the second application.
All those marks by the bolt recess and others visible on the assemble rifle I would expect to be filed or buffed before hardening. If it was possible to do after heat treat the only ones with raised metal I would expect would be final proof marks and such. If filed or buffed it would have take the final case hardening colors off. 
One thing I know for sure I was not there to see how it was done!
If someone wants to buy that pile of receivers and send them to me, I will do a high magnification analysis and give you all a more educated guess.   (You need to Login

Went back and looked at Mark's proof mark and it does look like a file was applied to it after stamping, maybe they did a second tempering to get color and slight corrosion resistance after it was stamped? What do you guys see on other proof marks?
A second tempering would be done at a slightly lower temperature. For those that do not know tempering is usually a second process done after the hardening. Double tempering is not common.