Being 10/79 it would have had the alloy wheels not the steel GTS rims. They had a different winch cup again. Being pretty much at the cease of Sandman production and the wrong trim code? I would think its not a Sandman but you need to get in touch with HHS. They are the experts.
It's got good options though and if it's in good order you could make a great van out of it no doubt.