On September 11, 2018, about 1637 central daylight time, a Maule M-7-235C, N969AW, was substantially damaged after the left, main landing gear collapsed at a private, grass airstrip near Floresville, Texas. The flight instructor and a private pilot-rated student were not injured. The airplane was operated by an individual under the provisions of 14 Code of Federal Regulations Part 91 as an instructional flight. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight that originated at Floresville about 1600.According to an inspector with the Federal Aviation Administration (FAA), the flight instructor and his student were practicing approaches. Upon landing, the pilots heard a loud popping noise, and the flight instructor performed a go-around. The student in the left seat could feel vibration under the floor board. Suspecting a blown tire, the pilots discussed how the airplane might handle during the next landing. The instructor flew the approach and landing. After a normal 3-point landing, and just before the airplane came to a stop, the left main landing gear collapsed. The FAA inspector responded to the accident site and examined the wreckage. He reported that the left wing sustained structural damage during the landing. The propeller struck the ground after the gear collapsed. The inspector observed that the left main landing gear inboard attachment bolt was missing. The bolt was later found along the path of the landing rollout. The bolt threads were undamaged, and the attaching nut was not located. The right inboard attachment bolt and nut were still attached; however, they were untorqued and loose, with only 2-3 threads visible past the nut. The 100-hr/annual inspection procedures in the M7-235C Maintenance Manual (MM) stated that, under the main landing gear inspection procedures, "The torque on the inboard attach bolts must be 240-300 inch pounds and on outboard clamp bolts 160-190 inch pounds." The airplane was fitted with upgraded high strength attach bolts. Maule Service Letter (SL) No. 66, revision B, dated May 2, 2006, addressed the installation of the high strength bolts. According to the SL, the torque for the outboard clamp bolts was the same as the MM inspection guide (160-190 inch pounds); however, the torque for the inboard attach bolts increased to 450-500 inch pounds. Service Letter No. 66 also stated, "Retorque bolts after 25 hours and per Maintenance Manual thereafter." The MM did not incorporate the new torque requirements for the inboard bolts. Following the MM only, this allowed the inboard bolts to be torqued to 240-300 inch pounds, or about 200 inch pounds lower than the requirement in SL No. 66. A review of the aircraft maintenance records revealed that the most recent annual inspection was completed in April, 2018. The mechanic who performed the inspection reported that he did not retorque the attachment bolts; he checked them by feeling them with his hands only. He did not reference SL No. 66 during the annual inspection.