Thats very impressive Paul, very interesting. I would have to say that as the groups are consistent in letter numbers, that each letter group may represent a particular thing or subject and might therefore be part of a daily report or at least a regular request or answer for information like unit status, strengths or position. These codes would be changed very regularly to avoid detection or de-coded, perhaps even daily depending on the likely threat of deciphering.....so you may never find the original code that this represents. Perhaps the unit history for the SAC might give you an insight, the message date is there to narrow it down.Its odd though that part of the message has been sent uncoded.
We were using similar codes with the Australian Army in the late 1980's.
If you do find out, please let us know.