While practicing three point takeoffs and landings in the tail wheel equipped airplane for a Federal Aviation Administration (FAA) required examination, the pilot experienced a loss of control when the airplane veered to the left after touchdown. The pilot stated that he tried to correct to the right but that he heard an abnormal noise from the tailwheel and then it locked. Examination of the accident site by an FAA inspector revealed that the airplane had run off the right side of the runway, hit a taxiway sign, traveled 50 more yards, and nosed over. Examination of the airplane revealed that its airframe was substantially damaged. The fabric was torn in numerous places, the propeller was broken, the forward left wing strut was bent, the right landing gear was broken and bent up under the fuselage, and the airframe exhibited evidence of torsional deformation. Further examination revealed no evidence of any preimpact failure or malfunction of the flight controls. No binding was discovered and the tailwheel would move freely to the left and right with rudder pedal application.