i did not know how to include pictures before, so the first edition of this topic was (appropriately!) blocked by the moderator. my apologies to all.
i stripped the splines out of my 1985 K100RS driveshaft a few weeks ago, and pulled the driveshaft and final drive off a 1992 K75 hangar queen to get myself back on the road. afterward i replaced the driveshaft and went back to the K100RS final drive, because the K75 final drive ratio was a bit too low -- my RPM's were about 10% higher at highway speed. this meant great low-end torque but more engine noise on straight roads. if not for all the straight roads in between the twisties, i'd've kept this K75 drive. however, i want the K100RS to remain stock.
note that the final drive and driveshaft are a matched set; the number of splines on the interface is different for the K75 vs. K100RS vs. K100LT. i'm selling a matched known-good final drive and driveshaft from a 1992 K75, which i just road-tested to las vegas and back in my K100RS.
$250 + shipping. i'll make a note here if it's sold.