The pilot of the tailwheel-equipped airplane reported that, he was on a familiarization/test flight with the pilot/owner. They had conducted 7 successful landings and while on final approach for the 8th landing, he entered a forward slip to steepen the descent angle without increasing airspeed, and pitched for landing. As the airplane touched down, there was no drift or yaw; however, during the landing rollout the rudder pedals "jerked," and the nose began to veer rapidly to the left. He applied right rudder pedal, but the airplane continued to yaw to the left and subsequently ground looped. The airplane sustained substantial damage to the rudder The pilot reported that the tailwheel chains and steering springs were intact, but the chains were not attached to the springs, and one of the tailwheel steering chains may have disconnected while maneuvering to land. The nut retaining the tailwheel spring was not located and the spring hole appeared to be undamaged.