Web info on the GCA Trucks is dismally thin. I found one picture of the inside, which was about as informative as The Sunday Sport.
As you say, chugalug2, a shame nobody seems to keep proper records. Perhaps CATCS at Shawbury has something, like an old copy of AP3357 (Manual of GCA), and could post some pictures. I shall try to poke them, if there's real interest.