I was recently given a Camillus, NY knife. It is stamped USN on the base of the blade. It is stamped Camillus NY on the opposite side. No other markings. I also have the leather sheath also stamped USN. They both look almost brand new, however I know that the former Marine Officer, who is now deceased, had it during the Vietnam War. I'm just trying to figure out how old this is. Any help would be appreciated.