Z was the British prefix for 15 cwt-3/4 ton vehicles. However, always a however, the American's use a short ton of 2000 ilb whilist the Imperial ton is 2, 240 lbs. So the max theoretical load of a British truck is 1680lbs, whilst the rated max load of a Dodge , from manual is 1500 lbs including driver and assitant. On a more practical level, put mine over a rated weigh bridge with canvas and fuel tank full, no load and got 2,400 kg. The other oddity is that British trucks seem to have pionner tols on the inside of the tailgate. Allows for full opening but can be a nusiance for load.
815-9 suggests a date of late 1942.