I'm sure that there are others that may specifically know better than I, but from what I've read over the years, the seat mount for the '85 and possibly earlier, is a different style mount and that most all bikes after that year use the same hardware and mount type... k75's, k100's and k1100's.
From what I see of the different bike designs, the bike/seat combo that has the side grab handles part of the seat mount and not the rear tail cover mount(up to 08/85), use different seat mounting hardware. I see this design only on certain K100 models/years both EU and USA versions.
Starting 09/85(ie model year '86) BMW changed the tail cover and mounted the grab handles to the tail cover, as well as changing the seat and mount hardware.
So in a nut shell, the seat you're trying to sell, should fit any bike with the passenger grab handles mounted to the rear tail cowl... but that's just my opinion from searching parts databases.. YMMV.
Anyone interested in this seat should continue to do your own research on this.