Mooney 20C instructor pilot reported a landing gear operational issue during landing on a training flight. The gear collapsed upon touchdown and stopped on the runway with aircraft damage and no injuries.

Narrative

I was conducting a Biannual Flight Review for Pilot A (His BFR expired several months earlier). After the ground portion of the BFR we proceeded to brief the flying portion and discussed the maneuvers and tasks we would be performing. I examined his log book and we discussed the operation and characteristics of the 1964 Mooney M20C. (Pilot A owns a 19XX M20C with electric flaps and gear that was down for repairs). Two of the things I stressed was the operation of the flaps and Johnson Bar Landing gear.After our briefing we performed the pre-flight inspection of the aircraft and found the aircraft to be airworthy with all required documents on board. We than Started the engine and received the ATIS information. After receiving taxi clearance we proceeded to the run-up area near [taxiway] 1. We performed the run-up and all checked out properly. When then received clearance to taxi to Runway XX via [taxiway] 2; 3 and were cleared for take-off with an east departure.After departure we climbed out to an altitude of 6;500 ft. and proceeded east towards the practice area. Over the city we continued our climb to 7;500 ft. During our flight to the practice area we transitioned to cruise flight. We then announced our entry into the practice area and proceeded to perform several maneuvers including stalls; slow flight; turns and emergency procedures. These maneuvers also included transitioning to landing configuration and lowering and raising the gear numerous times. The maneuvers were all performed satisfactorily and we returned to the airport.Ten miles east of the airport we contacted tower; told them we had ATIS information; and requested touch and goes. Tower cleared us to enter a right downwind for runway XY and to report five miles east.We reported five miles east and we continued our downwind. Approximately two miles east we proceeded to configure for landing. We went through the BCGUMPS check list (Boost Pump; Carb Heat; Undercarriage; Mixture and Speed-seatbelts). Reaching 120 MPH we lowered the gear and upon reaching 100 MPH we put in 20 degrees of flaps and reduced speed to 90 MPH. We received our clearance to for a touch and go on runway XY and were told that on the go to extend our upwind. During this time I went through the BCGUMPS check list several times visually checking each item. We turned final and reduced speed to 80 MPH and deployed full flaps. I did another BCGUMPS check visually. (Unfortunately I could not see the gear down light from the right seat; but did see the Johnson Bar in the vertical position) I DID NOT VERIFY THAT THE LARGE PLUG ON TOP OF THE JOHNSON BAR HAD LOCKED INTO PLACE. We flared out for landing and as we reached the runway; the gear started to fold into the belly of the aircraft as we performed a gear up landing. We slid down the runway several hundred feet and came to a complete stop. Damage consisted of a sudden engine stoppage and a prop strike. Also some damage to the under belly. There were no injuries except to my ego.One of my major take-a-ways from this incident; Don't assume anything! Verify! I should have physically checked to make sure the Johnson Bar had locked in place. A mistake; I can assure you; I will never make again.