Thanks Sandaro the engine number on my block is QR913866 and matches the engine number listed in the build record letter that GM Holden sent me.
All other vehicle details match up except the transmission code, the vehicle tag says M20 but the Holden letter says 3.74:1 4 Speed Manual (M22).
The vehicle does have a noticeably low 1st gear and I often choose 2nd when stopped at traffic lights.
So the vehicle was built with either an M20 or an M22 low ratio gearbox and a GV7 G80 low ratio diff.
Would that still be consistent with a Sandman van spec do you think?