You've got it wrong. 4850lb (HQ) or 2200kg (HJ-HZ) was ALWAYS the GVW for a V8 Sandman from start to finish. Only the 10/75 onwards XX7's had a lower GVW.

At least being a Pagewood HX ute you'll know if the body is original to the tags by the plant tracking number on the firewall. But I don't think there will ever be any way you will know for certain if it is a Sandman or not.