Much of that is wrong!

The number you have mentioned which is near the steering box is the CHASSIS number. Yes it should match the number on the compliance plate but it never matches the VIN of the vehicle (if the vehicle has had a chassis swap the numbers will not match). The VIN is on the long skinny tag. It ios the last bit of the VIN (called the PSN - plant sequence number) that vehicles are recorded by, but there is no correllation here to the CHASSIS number.

There is no MODEL ID on the compliance plate. The model information is on the BODY plate.

Searching GMH's records for the PSN will NOT shed any more light on the vehicle being a Sandman or not (note HJ van cannot be a Belmont, no such thing). There is no more vehicle identification information in GMH's records than what is contained on the vehicle's ID plates. The only additional information that may be there is:

Engine number as fitted at 1000kM service.
Transmission serial number.
Vehicle completion date (99% of the time this will be the same as the MM/YY on the compliance plate).
Mechanical options: Air, Steer, Discs, LSD, diff type plus other stuff already on BODY plate (Eng, Box, diff ratio).
Sale date.
Selling dealer.

The only Sandman specific thing you can tell from the records is if a vehicle is definitely NOT a Sandman, and you tell this by it being either a 3spd (HQ-HZ), a Kingswood (HX, HZ only) or it being a 6cyl (HZ only). However you can pick all this off the ID plates.