Alu adapter plates are bolted onto the back of Hepco & Becker side carriers in order to allow fitment of any hard side case from Hepco & Becker. These alu adapter plates are include in new Hepco & Becker and Krauser cases. Most people buy these if they have purchased a used set of cases or plan to use the same cases on more than one side carrier. 

These are sold individually so you will need to buy two for a complete set.