These reflectors are the nicest available and made by German masters, Busch & Muller. They are designed for fork crown and seat stay bridge mounting and supplied with bolts.
A safe bike is essential, and these reflectors can play an essential role towards rider safety if running cantilever brakes.
If a cable snaps, the straddle bridge is forced into the tyre and can result in locking the wheel up, and potentially sending the rider over the handle bars.
Reflectors prevent this from happening and give the rider protection from such issues. A small price to pay for peace of mind when descending that big hill!