1903A3 barrel

I have a spare barrel which is marked with "RA and the bomb plus the date
8-43", which I am familiar with, but it also has some other markings that I am not familiar with. There is an "A" near the chamber end and a "O" with a slash across it on the opposite side, plus there is "VIMU" stamped abou 61/2 inches from the chamber end. From the looks of the bore and chamber, I can't tell there was ever a round fired thru it. I hope my questions aren't too bothersome, thanks in advance for any info.