I'll admit I am not an expert on Washburn, especially once production moved to Asia. Washburn, as you may have already discovered, has no archive pre-1990...wonderful, eh?
I agree I never heard of a Washburn HB75...but there are HB models, HB obviously standing for Hollow Body. There are single p-up HB15s with that Florentine cutaway. Maybe that '7' was a '1'?
My guess is that yours has been modded, originally maybe it was a J series, like the 3 [which has the Florentine cutaway]. However there are several issues with that assumption, mainly the pickup locations, the neck inlays, head inlay, the pickguard and tailpiece do not look like that model. More likely it is a J-10 with a different headstock inlay, and modded pickguard but the rest is pretty much there.
I hope this helps at least get you on the trail.