Here is a scan of the relevant pages from
Data Book of Wheeled Vehicles: Army Transport 1939-1945. Turns out they were delivered under S.M. 2227, 2228 and 2229.
(click on image for large size scan)
From David H.'s
British Chevrolet Contracts, we learn S.M.2229 was allocated census numbers L777093 to L777366. This means 274 Tippers were supplied to the British. Shown below is a restored Chevrolet Tipper, pictured a few years ago at Beltring. No idea if this was originally supplied under S.M.2229, though.
Source:
Ferdinand Hejl's photo site