Re: 1903 Springfield Mk 1
RA is the mark the Raritan Arsenal put on rebuilds. The P probably is an inspectors mark.
Raritan rebuilt M1903's during and just after WW2, so your rifle may or may not have all original parts. The mark merely means it went through the arsenal. If there was nothing wrong with it, they probably didn't actually "rebuild" anything. Just marked it and sent it on it's way.