Rafterman

Joker seems to make it a mission, initially, to see to it that Rafterman sees no combat action. He jokes that Rafterman's mother would be particularly pissed off at him if he didn't do all he could to prevent him getting killed, almost as if they knew each others' families back in high school or something. I know that Rafterman dies in the novel, but I'm curious if they knew each other before going to Nam or if Joker was just playing a big brother role for reasons the film doesn't indicate.