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
This scooter, however, is unlikely to be a moped. It may be a smallframe Vespa, but i gather that, with the load it is meant to carry, it would be based on a 125cc model. It may be a largeframe, I can´t tell from the picture since I am not very familiar with Vespas before 1990, and if so it may have a 125, 150, 160 180 or 200 cc engine. Or, it could have been custom built for the army with any old engine setup.
Either way it is one of the craziest Scooters I´ve seen, and I´d love to do a replica - all though I very much doubt that I could get it road legal here in Norway...