There is no unique trim or GVW for Sandman in HJ. They got the same trim as whatever the base vehicle had (Holden or Kingswood). The GVW was the same as the base vehicle for most of HJ (2155kg for 6cyl and 2200kg for V8). Only the late XX7's in HJ got a reduced GVW and then it was common with XU4.

The only Sandmans you can pick by GVW are 1978-9 complianced HZ XX7's, and the only ones you can pick by trim code are some HX and some HZ. These are the ID I can confirm as Sandman from ID alone.

Dashes, courtesy lights, seating capacity, GVW, headlining etc are all good indicators but are no guarantee as they are all options on non-Sandmans.