On June 8, 1996, at 1130 central daylight time, a Weatherly 620B, N2004L, was substantially damaged during landing at Moore County Airport in Dumas, Texas. The commercial pilot was not injured. The aircraft was registered to the First National Bank of West Texas and operated by a private owner under Title 14 CFR Part 137. Visual meteorological conditions prevailed for the local aerial application flight which had originated at 1030. A flight plan had not been filed. The pilot reported the following information in the Pilot/Operator Aircraft Accident Report. When the aircraft returned from an application flight for landing, upon touch down, it began to nose over and veer to the right. The pilot "pulled on the stick and got on the left brake," but the airplane continued to veer right until it exited the runway. The pilot stated that "when it got to the dirt it dug in and flipped over onto its back (top)." After the pilot got out of the airplane, he found the right main gear perpendicular to the longitudinal axis of the aircraft and the right main gear torque link bolt broken. Post-crash examination by the manufacturer revealed that the right main gear torque link assembly had been improperly assembled during a field maintenance procedure, approximately 50 flight hours earlier.