This 1994 911 Turbo S “Flachbau” is one of the rarest and most sought-after Porsches of its time, combining the best of the classic air-cooled, single-turbocharger, rear-wheel-drive Turbo era with distinctive looks and undeniable exclusivity.

So it is perhaps surprising to note that the model owes its existence to a quirk of Porsche production history: As the automaker wound down production of its 964 Turbo in late 1993, and with the 993 Turbo on the horizon, a final batch of uncommitted 964 coupes was sent over to Porsche Exclusive for conversion to "S" specification.

Consequently, all were equipped with the "Works Increased Horsepower" X88 engine option, uprated with a larger, single KKK turbocharger with increased boost, a more efficient intercooler, modified cylinder heads with larger ports, increased-capacity fuel injectors, and more radical camshafts with advanced engine timing.

The M64/50S X88 was an air-cooled flat-six which delivered 385 horsepower at 5,750 rpm, 25 more hor...