The instructor was giving the pilot a type endorsement and this was to be the final circuit. It was thought the gear was extended on downwind with appropriate cockpit indications to confirm this. Just prior to touch down the gear horn sounded and the instructor called for a go around. Power was applied as the aircraft contacted the ground. The landing gear legs were still up after the accident but the gear doors had been open and were damaged. This was consistent with a gear down selection being made at the last moment.