The following information provided by Gordon M of the G503. Thanks Gordon.
The British had a standard workshop truck, the Austin 6 x 4, 3 ton, and needed some extra numbers, so they bought these chassis / cabs from Canada.
I think they were US built chassis-cabs, with Canadian-supplied Welles-Thornton double rear bogies. Since they were finished in Canada they could be supplied to the UK without bending those war material export regulations. The Ashby clan in the south of the UK have a surviving example, but the cab was pretty far gone so I believe they got a replacement cab from Dave Butler at Vintage Power Wagons.
They all got wooden workshop truck bodies with an overheard gantry beam that could be run out the back to work as a crane, but as trucks with no front wheel drive and low-capacity cranes they weren't a lot of use tactically. Got a small image of one stuck in mud while testing but no way to post it here, and I think one or more got captured by the Germans (in north Africa?) and re-marked when under new ownership too.