Portions of this short video show that John found show the rapid assembly of a jeep. It’s easy to see how this evolved into assembly competitions.

“The King Inspects Cadets In Training (1948)
Aldershot, Hampshire.

General view of tanks and infantry taking part in a cadet training manoeuvre at Aldershot. Back shot of King George VI and party in foreground. Pan from the King watching manoeuvres to tanks moving across plain. Tank being loaded onto lorry belonging to a tank servicing unit. Troops assembling jeep in record time. King with an army officer viewing assembly off jeep. Four soldiers hard at work assembling front portions of a jeep. King and army officers watch cadets carrying out a wireless training exercise. Army wireless equipment with back of radio operator at right of screen. King and two army officers seated in jeep enjoying private joke as jeep, which is mounted on tracked army servicing vehicle, moves away from camera.”