The silver J-shaped piece on each caliper bracket rotates through 360 degrees to provide a brace against the inside face of the seat stay or fork. The rotation allows adjustment to accommodate different boss/tube centre offsets. I have fitted these brakes successfully to a mountain bike frame that was designed to take Shimano Deore cantilevers identical to those shown in Richarcareys photo above.
Hi Foz - if you still have the Magura HS66 brake set I’ll buy at that price. Never bought in Retro Bike before (occasional lurker) but presume you will pm me with email address for PayPal etc.