I've contacted the owner and he's sent another couple of photos, enough to see that it is indeed a DMS 370 but nothing more. It's a rolling wreck. Still has the DMS style exhaust on it. Looks to have the mk2/3 style swingarm and canted shocks. In the only comment I've read Dick Mann stated that he got these and the Honda made in '79, which is odd as the swingarm and frame design are different. I can only assume he had the 370 version made in early '79 and the Honda late: but then in that VMX 10 byline he also stated he made 25 of each and according to this thread he recently stated he made less than 10 DRs. Anyway the Honda and mk4 TT500 frame appear to have the same swingarm and a very similar shock placement and frame design and mk4s were made up to 81. I've located four of them. One a show queen in the current VMX72 wrongly ascribed as a mk2, one turned into a roadbike, one in Portland with a cracked headstock and a bodged repair, and mine. Mk3s seem even rarer.
Anyway I'm hoping to go and see the DMS 370 this summer, or at the very least get some better shots of it.