On July 22, 2006, about 1230 eastern daylight time, a Cessna 550, N466SS, registered to and operated by Capital Buyers of Delaware, Inc., experienced partial collapse of the right main landing gear while taxiing after landing at The Florida Keys Marathon Airport, Marathon, Florida. Visual meteorological conditions prevailed at the time and an instrument flight rules (IFR) flight plan was filed for the 14 CFR Part 91 corporate/executive flight from Dennis F. Cantrell Field Airport, Conway, Arkansas, to The Florida Keys Marathon Airport. The airplane was substantially damaged and the commercial-rated pilot and three passengers were not injured. The flight originated about 0930, from Dennis F. Cantrell Field Airport. The pilot stated that after takeoff the landing gear retracted normally and the flight proceeded to the destination airport. While en-route to the destination airport and descending from 19,000 feet to 15,000 feet, he heard a sound, and one of the passengers who is the owner advised seeing a lightning strike. The flight leveled off at 15,000 feet and the only discrepancy he noted was the failure of the No. 1 communication transceiver to transmit. The flight continued to the destination airport and he cancelled his IFR clearance. While turning onto the downwind leg flying between 165 and 170 knots, 15-18 degrees of left bank, with approach flaps extended, he lowered the landing gear using the normal method and he "noticed an extra clunk" sound. He noted 3 green lights indicating all landing gears were down and locked turned left base and final. He again confirmed 3 green lights prior to a smooth landing. During the landing roll he activated the thrust reversers and turned left onto a parallel taxiway. He then turned right onto a ramp where the airplane was to be parked and he felt the right wing drop and thought the right main landing gear tire was deflated/blown. He secured the airplane and noticed that the right main landing gear was displaced outboard. He then examined the taxiway and noticed that a mark associated with the right main landing gear tire was noted where the airplane was taxied off the runway onto the parallel taxiway. He further reported that there was no abnormal indication of the landing gear after extending the landing gear for landing or while taxiing prior to the partial collapse of the right main landing gear. The airplane was repaired before NTSB notification, which occurred on August 2, 2006. Maintenance personnel later reported to the NTSB that during recovery of the airplane, a stud (P/N 6541204-5), which secures the main landing gear actuator assembly side brace to the main landing gear outer cylinder, was not in position and was not located for the right main landing gear. Prior to NTSB notification, components of the right main landing gear were sent for inspection/repair to Cessna Aircraft Company, located in Wichita, Kansas. Following inspection/repair, the components were sent to a Cessna service center located in Orlando, Florida, for installation onto the airplane. Following NTSB notification, the right main landing gear outer cylinder (P/N 5541320-6) was resubmitted to Cessna Aircraft Company for further examination. Examination of the actuator assembly attach stud tapered hole revealed no unusual markings and the inside diameter was within limits.