On November 9, 2003, approximately 1600 Pacific standard time, a tail-wheel equipped Cessna 150C, N7934Z, registered to and operated by the pilot as a 14 CFR Part 91 instructional flight, collapsed the left main landing gear during the landing roll at the Toledo-Winlock Airport, Toledo, Washington. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local flight. The aircraft was substantially damaged and the flight instructor and commercial pilot were not injured. During a telephone interview and subsequent written statements from both the commercial pilot and flight instructor if was reported that the purpose of the flight was instruction for the commercial pilot to attain the tail wheel endorsement and a flight review. The commercial pilot just purchased this aircraft and this was the first flight. Both pilots reported that after accomplishing air work, the flight returned to the airport to accomplish landings. The instructor stated that the commercial pilot was at the controls for the full stall landing to runway 23. The aircraft touched down straight then started a slight turn to the right. The commercial pilot called for the instructor to take control, which he did. Corrective inputs were unsuccessful to realign the aircraft to runway heading. The left main landing gear eventually failed and separated from the structure. The aircraft came to rest on the side of the runway with the left wing contacting the surface. Inspection of the aircraft for damage revealed that the fuselage was twisted at the door post and there were wrinkles in the fuselage skin, the firewall was damaged, and the left main landing gear was pulled out of the gear box. Inspection of the left main landing gear by a Federal Aviation Administration Inspector from the Seattle, Washington, Flight Standards District Office, found that the wedges, part number 0410023-2 (2 each) were missing. The inspector further reported that the retaining screws were 1/8 of an inch further out than the right side, where the wedges were still in place. Maintenance records indicated that this aircraft had been converted by a supplemental type certificate (STC) to a tail-wheel configuration just prior to the commercial pilot's purchase of the aircraft and his first flight. An annual inspection had been accomplished on April 28, 2003, and approximately 9 hours had been accumulated on the aircraft since the inspection.