Terry, the only 904 I can ID is 66-16904. There may have been a 66-15904, but no specific records of it available. Any chance you may have a photo of the tail number? There are better records on ships after your time than for during and before you. If it was 66-16904, it actually went into commercial service after Vietnam, and was destroyed in a crash in May 2003 when an empty sling cable flew up into the tailrotor.