I have been researching part numbers and model interchangeability of seats on Max BMW's parts fiche. It appears that BMW had four different double seats for the K bikes. First is the "low" seat that slid on and didn't have battery side covers.
Second is the "dual bench" seat that I think is what you are looking for. It is pretty simple with a modest step between the driver and pillion. This is what I think was the "standard equipment" K bike seat. Third is the "comfort" seat. That is what you have. Then there is the "dual saddle" that has a higher step between driver and pillion, and thicker, softer padding.
Except for the "low" seat they all appear to be based on the same hinge arrangement, if not the same seat pan. It appears that different foam was molded onto a common pan to make the different seats. This is so the factory didn't have to build different frames to accommodate different seats. It also made it possible for dealers to upsell the standard seat on the showroom floor. I have three seats, a dual saddle on a K75, a dual bench on a K100, and the pan from a K1100LT dual saddle. All three are built on the same pan with the same hinge arrangement.
There is also a lot of interchangeability of seats between the different models. The biggest limiting factor here appears to be that the earliest K bikes had a different handle arrangement from the later models where the handles were incorporated in the rear seat coaming.
What I have been able to see from limited experience of a K75rt and a K100rs4v is that there appears to be a common hinge arrangement on the "classic"(late 80's and 90's K75's, K100's, and K1100's) models. There does appear to be some difference in seat pans, as some will not work with the under seat tool tray and cover. This is true of my two 90's vintage bikes, neither of which came with the tool tray.
Hope this helps.
