Mark,
I was watching Billie's DVD again last night and in one of the frames there is a shot of a revetment with the number 533. I can't remember whether that was Billie's ship or not. I think Gene Touchstone's was either that or 536. I know that 498 was a replacement ship which arrived in Dec. 1968. It came in the same time that my new ship 488 arrived.
As ships were replaced older A/C would move to the newer ships. Marty may have moved from Bob's ship to Billie's when he got 297. Bob's ship was 603 during our time in the Plei-Trap. He was a bullet magnet. Of course flying with "Hud" was the reason for that. Ex-special forces man, who didn't fear anything. Great pilot!
I will continue to dig. Maybe someone will pop up with the answer.
Ron