To the original message; none of the google results show a yaw damper. The first picture tells you about the 737 yaw damper, but shows one of the PCUs I think. The yaw damper is in that same area. It depends from aircraft to aircraft how big it is, but it's basically not 1 item. Older aircraft tend to have more mechanical linkages whereas modern aircraft calculate the amount of rudder deflection in computers. The yaw damper is closely connected mechanically to pilots pedals, rudder ratio changer, autopilot actuators and the trim/neutral offset cam. Rods and springs make sure that e.g. the rudder pedals do not move when yaw damper makes inputs etc...
I've uploaded a technical drawing for you
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