James, like so many other B'ham Motorcycle Manufacturers, stayed within the City limits, usually moving to close by buildings if at all. WW II damage was shrugged off. Unlike Triumph whose original City Centre Factory was completely destroyed, hence the move to Meriden. James were never a 'biig' player in the game, unlike Ariel & BSA, but persevered, turning out commuter machines and big, heavy, workhorses. Their V Twin engine powered the three wheel delivery vans so beloved by the Co-Op.
They kept going into the late 60's making a reasonable competition 'bike, (off Road), but failed when Villiers stopped making motorcycle engines. Many 50's & 60's James are used to compete in modern pre 65 Trials & pre65 Scrambles.
As has been said, James switched to War production, but unlike some, kept up the 'bike role with the ML (Military Lightweight) model for air and sea borne assault troops to use. Known as the clockwork mouse, they were an improvement on the Brockhouse Corgi Parachute bike. (See National Motorcycle Museum for examples).
On the subject of reversing 2-Stroikes. Villiers used a simple system of overadvancing the spark thus the engine would start up 'backwards'. This was used in the Bond Mini-car, but could also occur on Motorcycles if the points had not been set and tightened up correctly. Usually, suffering loss of power by firing at TDC, the engine died at a junction. Kicking started it again, but, unbeknown to the rider, the engine was running backwards. A big handful of throttle and... reverse wheelie !