The purpose of the flight was for the pilot-under-instruction to gain endorsement for constant speed propeller and retractable landing gear operations. This was only the second flight on type for the pilot-in-command and the first for the pilot-under-instruction. The pilots stated that the pre-landing checks had been completed and that the landing gear down and locked light was illuminated. The light was rechecked on final approach to the runway. The pilots reported that the touchdown seemed normal but the aircraft soon adopted a left wing low attitude. The propeller then contacted the runway surface and the aircraft settled on to its underside before skidding to a halt. A thorough examination of the landing gear system revealed no fault. The type and location of abrasion damage to the nosewheel doors, and the lack of damage to the main gear fairings and brake assemblies, indicated that the doors were closed when the underside of the aircraft contacted the runway, ie. the landing gear was in the retracted position. It was noted that the red and green post lights on the instrument panel which indicate high and low vacuum pressure were of the same type as the landing gear position lights. The green vacuum light was seven centimetres from the green landing gear down light in approximately its two o'clock position. It was considered possible for the crew to have mistaken one light for the other.