I think 1991 or '92, although it's hard to be sure. My 1992 has vertical dropouts, but the model pictured in the 1992 catalog is shown with horizontal dropouts. Could be the catalog was shot using preliminary spec'ed models, or maybe they were in the middle of a running change from horizontal to vertical, and were just using up the last of their stock.

And then again, I just looked at my '93 catalog, and the pictured front suspension Stump has vertical dropouts, while the rigid fork Stump has horizontal.