The earliest pads had a piece of thread sewn in the middle to hold the leather down on the felt. By the 1920s they used a metal rivet through the middle to firm up the pad. After than a metal washer (resonator) was under the rivet to boost sound. Next was a plastic domed resonator screwed or riveted from the underside. Martins came with rivet pads but most people replace with resonators either metal or plastic. There is a lot of debate on which to use (I use brown plastic domes). Whatever you use, get the thin ones as fat pads will never stay correct on a Martin.