Here are a few:

Kingswood Door Trims.
Carpet.
Radiator Grille should be all black.
Wheel Arch Moulds.
Door Moulds.
Seats should have hard board backs (Sedan).
Full Length Roof Lining (Din't happen in HQ).
Chrome Step Moulds/Scuff Plates (Premier/Statesman).
Door Handle Scratch Plates (option).
Chrome Tail Light Surrounds (option).
WB Windscreen.
Radiator Cowling (Air Conditioned Only).
Rear Venetian (aftermarket).

Sure it can be debated that some (or most) of these are improvements and I'm not bagging the changes, just pointing out they are not factory spec.

Nobody knows the numbers on HQ Sandman's produced (not even Holden) so everything is speculation or a plain guess.
Logic would suggest the number was quite a bit higher than 100 units for sure. I'm with Absinth on that one. But '74 wasn't the hight of Vanning it was around the enthusiastic start.
The peak of the Van craze would have been '76-'78 with the highest standards happening in the '80's.

The numbers on the HQ SS were what? 1600 and than 1200 or similar. It will be under that count for sure. They made around 250 VF Sandman's in Ute's and Wagon's so I think that would be closer to the mark and quite possibly more!