Vespa MP6

After the "Paperino" MP5 was rejected, Corradino D'Ascanio applied his aeronautical thinking and solved every complaint at once. He ditched the tall central tunnel so riders could step through easily, moved the engine to the rear next to the wheel, added the mono-arm front suspension (borrowed from aircraft landing-gear thinking so wheels could be changed like a car), and gave it that unibody monocoque with the wide rear cowls housing the engine. Enrico Piaggio saw the finished prototype, heard the buzzing engine and reportedly exclaimed "Sembra una vespa!" ("It looks like a wasp!") — and the name stuck.
The MP6 went into production almost unchanged as the Vespa 98 in April 1946, with roughly 2,500 built in that first year. So visually the MP6 and the earliest 98 are nearly identical — the giveaway details on the true prototype are things like the headlamp mounted low on the front mudguard (fender) rather than up on the handlebars, which came later.