To mount the TK50 top case on your K75, K100 or K1100, you will first of all need the luggage rack that attaches to the tail section. With that installed, you will also need this mounting plate.
If you had or still have the old 22 liter top case (original BMW) then you may already have this part! It is the same bracket that was used for the original BMW tip case. It slips into the luggage rack. The foam on the underside helps to protect the paint and also keeps tension on the plate.