For conversion from a side-pull brake to a dual-pivot brake system. When converting, the mounting hole in the frame need to be widened from 6 to 8 mm to make room for the recessed nut. The rear triangle leaves little to almost no space to insert a drill however, to expand the mounting hole. This adapter enables you to expand the 6 mm mounting hole from the other side, widening both sides to 8 mm. The adapter then reduces one of the holes back to 6 mm. The system is tested and works flawlesly, as is visible in the pictures.
Do not print in PLA, as the force the tightened nut exerts on the washer would cause it do deform.