The 43 Group is a British Anti-Fascist group set up by Jewish ex-servicemen after WW2 when they returned to London to discover Oswald Mosley and his fascists, fresh out of jail, had started up trying to re-build the British fascist movement.
The name came from the number of people in the room at the founding meeting, or so the story goes. They fought the fascists on the streets and Moseley was forced to go into exile in France. An attempt by Moseley to come back to Britain in the early 60s was seen off by the 62 Group formed in the 43 Group's image but named after the relevant year this time.
