It's an internal part of the KSU. The interrupter is a motor-driven lamp flasher. It has cams inside that close various sets of contacts at different rates per second. One was 60 IPM for incoming call flash, 120 IPM (wink) for hold flash, and then another set of contacts that closed one second on, four seconds off for ringing voltage. It is housed in a clear plastic case and plugs into a dedicated connector in the KSU or card rack. Later on, solid state interrupters were developed, but never became as popular as the motorized ones since they lasted almost forever!
