The flight instructor reported that while on downwind in the traffic pattern, the airplane began to yaw to the right. He immediately took control of the airplane from the student pilot and applied full left aileron and rudder, which reduced the yaw, however, the airplane still would not fly straight. He performed an emergency landing to a field, but shortly after touching down, the airplane nosed over, coming to rest inverted. The airplane sustained substantial damage to the vertical stabilizer and rudder. A postaccident inspection found that the forward vertical stabilizer attach bolt (P/N AN4-5A), nut (P/N AN365-428) and shim (P/N 0430010) were missing. (See figure 1) As a result of the missing hardware, the vertical stabilizer became unseated and bent aft in flight, which caused the uncommanded yawing motion. The hardware was not recovered. Figure 1. Fin assembly. The pilot reported that he was unaware of any recent maintenance in the affected area.