The context is the formal organization of Special Operations forces with the Department of Defense in the post World War II period. the USMC chose not to commit forces to Special Operations force structure until recently.
F Co, was F Company, 2d Marine Special Operations Battalion. Since 2014, this battalion is now designated as the 2d Marine Raider Battalion, linking to the tradition of the Marine Raiders organizations of World War II.
Back when, I was TAD to H&S, 3rd Force Recon when they were at Camp Ono on Oki, for a month, for some crypto stuff.
(This was back when crypto was still mechanical. We would pound out traffic on drums...)