Thats unfortunate, I was of the opinion that late HZ would be a problem to identify, yours being 2200 GVW.
That said, it really looks like a sandman, with all the identifiers you showed, I would say it is extremely likely a genuine Sandman.
As is always said, take heaps of pics as you go with the van, & the original factory records may turn up one day.

Have you checked out if theres the cut outs in the lower apron for original spoiler? & the fuse holder for the twin light front?