The proliferation of smartphone technology and social media at the start of the 2010s meant no manufacturer’s secrets were truly safe. Car enthusiasts who found themselves in the right place at the right time could reach into their pockets and snap a picture of the disguised “mule” they had stumbled upon, sure to send the internet into a frenzy. Guaranteed to be the most newsworthy were the heavily modified prototypes sneaking around the roads of Maranello.

Tying into the deployment of Kinetic Energy Recovery Systems and hybrid drive in Formula One, the turn of the new decade also heralded a step change in the powertrains that Ferrari, McLaren, and Porsche were developing for their forthcoming, electrically assisted flagship hypercars. While computer simulations could take engineers so far in their research, eventually rubber needed to hit the road. For Ferrari, that meant devising three cycles to test the nascent LaFerrari halo car, which was assigned the internal project code “F150.”

From the first phase of real-world development came the car offered here, Prototype M4 (also known internally at Ferrari as “F150 Muletto M4”). Evidently employing the same Tipo F142 platform as the critically acclaimed 458 Italia, this car had its aluminum chassis modified to accommodate the hybrid F140 V-12. On its own, the naturally aspirated engine—conceived for the Ferrari Enzo of 2003 and still powering the current Purosangue and 12Cilindri models more than two decades later—generated 789 of the LaFerrari’s eventual 949 horsepower.

The view from inside the Prototype M4 was mostly conventional 458, the cabin pairing black leather seats with tan carpets and a production-standard instrument binnacle featuring a white tachometer with an indicated 9,000-rpm rev limit. But the dashboard toggle switch and high voltage warning sticker placed over the Cavallino badge in the center of the steering wheel alluded to this car’s experimental credentials.

Outside, however, there was no mistaking this machine as anything other than a Ferrari skunkworks creation. The exposed rivets, overextended exhaust pipes, absence of wheels center caps, side hatches to access the powertrain, and mismatched brakes front to rear were mere details compared to the striking matte black livery, enormous front air intake, additional cooling vents on the trunk lid, and high-rise air intakes. The headlights, taillights, mirrors, and Ferrari shields set into the front fenders were among the limited number of 458 exterior components to not receive a dramatic makeover.

The first phase of LaFerrari prototypes completed their service between May 2011 and December 2012 and were spied testing at Fiorano and on the surrounding roads. With the full production car arriving in 2013, these development mules were offered by the factory to the most loyal of customers. This was with the understanding that the car was not homologated and therefore could not be registered for road use, nor could it be used on public race circuits.

Although this LaFerrari Prototype M4 has been retired from frontline duty and cannot be road registered, it is nevertheless an incredibly intriguing and significant piece of both Ferrari and wider hybrid hypercar history. As such, it is sure to draw a crowd whenever it is displayed and would make for a wonderful curio as part of a marque-themed collection.