That's a moped armed with a 75 mm recoilless cannon, called the ACMA Troupes Aeról Portées Mle. 56, used by (who else?) the French Military!
Deployed with their airborne forces, this was essentially a militarized Vespa scooter outfitted with a 75mm recoilless rifle. Five parachutes would carry the two-man gun crew, weapon, ammunition, and two scooters safely to earth, and the men would load the weapon on one scooter and the ammo on the other, then ride away. More impressively, the recoilless rifle could be fired effectively on the move by the best of the gun crews. Total cost? About $500 for the scooter and the recoilless rifle was war surplus. Were they successful military machines? Well, the French Army deployed about 800 armed scooters in wars conducted in both Algeria and Indochina.
Dark Roasted Blend has the scoop: Link