All,
The codes I referred to are listed for all airports/dirt strips etc. listed in the Tactical Aerodrome Directory issued by the USAF 7651st Aeronautical Chart and Information Squadron in San Fran. I think they published one every month.
May not have been a relevant document for helo ops but was the bible for tactical airlift (C7, C123 & C130). Major aerodromes also had an ICAO designator such as VVSD for Da Nang.
Only other codes I can think of would relate to radio nav aids such as Da Nang TACAN Ch 77 (DAG), although the Da Nang VOR (when working) was DAN.