These rubber connector pairs began being used in the late 1960s, as a crude attempt at weather-sealed connections. They were primarily used on motorcycle models that are fitted with Capacitor Discharge Ignition (CDI) systems, but also saw use on other models (primarily two strokes). This included the entire run of Kawasaki 500cc H1 and 750cc H2 triple cylinder two-strokes, plus several models of Suzukis. These rubber parts have been unavailable for perhaps a couple of decades. Similar parts are available elsewhere, but the material is not correct (it's more like a hard plastic, rather than rubber) and they are much harder to use.