Revealing mistake: When the mail arrives in the compound, in daylight, everyone has 3 shadows. (00:00:50)
Visible crew/equipment: After Hawkeye and Trapper tell Frank that they made up Pioneer Aviation to help him look foolish, when Hawkeye picks up Trapper's duffel bag, as the shot pulls back the chalk T-mark can be seen on the floor where Hawkeye had been standing.
Answer: It is actually Cencompac for Central Command of the Pacific. It is like the headquarters for all military activity in the Pacific Region (Japan, Korea, Okinawa, etc. The newer version of that is United States Pacific Command (USPACOM).
EMTurbo
And the abbreviations were used mainly by the Navy. Especially CENCOMPAC.
Https://en.wikipedia.org/wiki/United_States_Indo-Pacific_Command.