AF

"AF", a rather solid candidate for its origin might be from WWII, as per dialogue hereby portrayed in the TeleVision-series: "World War II in HD Colour" by 𝒥𝑜𝓃𝒶𝓉𝒽𝒶𝓃 𝑀𝒶𝓇𝓉𝒾𝓃, S1:E5: "Red Sun Rampant "at time stamp: 31:26 - 32:14; "Target AF"... rather the mysteriously unknown meaning of "AF". It took the US approximately north of 48 seconds to figure out such meaning: 32:14: "Admiral Chester Nimitz now knew exactly..."

Nauticans: "they had no ID where Target AF was"