I have an early 90's frame with this type of brake boss, it does not have the usual three holes, it has a raised crescent shaped lip presumably to fix against a certain type of brake... any one know what brake would work with this?
You're not giving us much info to work wtih. Mountain or road bike? Is the photo of a seatstay or chainstay or crossbar? Or fork? Etc. People here can answer your question, but most of us need more info to work with.
It looks to be a standard canti boss mount -- with one exception -- there is a flat metal washer that is held in place by the boss. Three other holes in this washer are for the brake springs. The crecent shaped protrusion keeps the washer from spinning from the force of the brake springs.