The crew noted that the nose and main body gear indicated up and locked after normal gear extension. The crew used the alt gear extension procedure, which was unsuccessful. The aircraft landed with the nose and main body gear retracted when the flight reached the min fuel state. Examination of the aircraft revealed that the selector valve for the affected gear was in the up and locked position, and the bolt which connects the valve arm to the cockpit control cable system was missing. The bolt for wing gear selector valve arm was found partially backed out; the nut and cotter pin were missing. The aircraft was delivered to United in Jan of 1990 and had 2,952 flight hours and 402 cycles. Detailed review of the maintenance records proved that United had not performed any maintenance on the landing gear system. A second alt gear extension procedure involving de-energizing the hydraulic system for the affected gear is not published in the crew manuals, taught during initial training, nor available on any checklist.