During the 1930s and 40's the police in London continued to use morse radio transmissions to communicate with patrols. They used a variety of cars and vans, some with receivers only, and others with the ability to transmit and receive.

The recieve only cars had to stop and telephone New Scotland Yard in order to acknowledge calls or send messages. This could be difficult in situations such as a pursuit, where there was no time to stop and find a phone. One suggestion, which, understandably was never implemented, was for the patrol officers to write their message ona piece of paper, place it in an envelope, and when they passed a beat constable they would throw the message out of the car, at the same time sending the letter "P" in morse code, twice on the car horn. (ie beep beeeep beeeep beep). The beat officer would then retrieve the message and phone it through to HQ.

Radio equipped Talbot patrol car - London, 1936 - Crew of two plus wireless operator in the back