The "Heroes" from Stalag 13 are communicating with a British submarine. The Submarine call sign is "Royal Navy 371". This is likely the pendant (hull) number identifying the type and specific ship. Numbers 311P to 399P were for "T" (Triton) class submarines.
The sign outside of Hogan's barrack is legible in this episode, it reads: "VERBOTEN. 1. Strict orders have been given to the German troops around Brussels to shoot any civilian cyclist. 2. People who, after the fifteenth of December, are still in possession of carrier pigeons, as well as all other persons, who by signals or any other means, cause annoyance to the German military interests, will be judged by courtmartial.
BRUSSELS, DECEMBER 14, 1944. The Chief of Military Police. H. HEINRICHS, Captain"
Stalag 13 is in radio contact with a submarine that relays the messages to "London G-2", which is a designator for military intelligence. G-2 is a staff section of division or higher units. Military Intelligence is a military discipline that uses information collection and analysis approaches to provide guidance and direction to assist commanders in their decisions
The British submarine commander, apparently at sea, and the British intelligence officer in London both have the same exact teacup.
The chimp has a 'uniform' on with French sergeant stripes. Corporal LeBeau is agitated that the chimp out-ranks him.