I did a search for prwr and found that this question has been going on
for years and I could find no definitive answer.
According to my search three things are possible (1) prwr means prewar.
This seemed to be disregarded immediately because it was found on 1910
census and how would anyone know that a war was coming ... hence pre
war makes no sense. (2) pr wr could mean prisoner of war, and (3) some
individuals found grmy prwr on census' >from people they were sure were
from Russia. Some think that it may be an early means of some machinepresorting code.
However, I am intrigued and I will keep searching.