On August 17, 2004, approximately 0930 Pacific daylight time, a Universal Moulded Products Monocoach, N11872, experienced a gear collapse during the landing roll at Pullman-Moscow Regional Airport, Pullman, Washington. The commercial pilot and his passenger were not injured, but the aircraft, which is owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which departed Porthill, Idaho, at 0745, was being operated in visual meteorological conditions. There was no report of an ELT activation. The aircraft was not on a flight plan. According to the pilot, just as the aircraft's two main wheels touched the surface of the runway, it suddenly jerked to the side and started to veer from the runway heading. He therefore immediately added power and pulled the aircraft off the ground. He then climbed out to an altitude where he could visually inspect the gear legs and evaluate his situation. When he looked at the gear legs, he discovered that the left main gear strut oleo had come out of its housing, and the gear strut structure was hanging from the aircraft by two of its attach points. He therefore circled the area for awhile, waiting for emergency vehicles to get in place at the airport prior to his attempt to land on the remaining main gear. Once the equipment was in place, he landed on the grass alongside of the paved runway. The aircraft touched down on the right main gear, and the pilot kept the other gear leg elevated as long as possible. Eventually, as the aircraft slowed, the left gear strut dug into the terrain and collapsed. A post-accident inspection of the gear system determined that the landing gear shock strut retaining nut safety clip (snap ring) had failed, thus allowing the shock strut retaining nut to back off of its threads. Without this retaining nut in place, the oleo piston was free to drop out of its housing, leaving the landing gear only partially attached to the fuselage.