When it comes to Ford’s stellar pony car the Mustang, it’s really hard to improve on a solid foundation – rear-wheel-drive, V8 engine and a manual transmission, packed in a two-door coupe or convertible. But Carroll Shelby in the mid ’60s found a way, and his way still resonates today with Ford.

It’s why the GT350 is still in production after more than 52 years (though to be fair, it did take a hiatus.) It’s also why we may see a GT500 returning in the future. But before we talk about another pre-production Shelby, let’s talk about the one we see before us today.

If you’re into low production numbers and that sort of things, well, this one might just tickle your fancy then – it’s a one of one, after all.

That’s because it’s the first, pre-production 1966 Shelby GT350, and it features some seriously trick one-off changes from any other Shelby. Backed by documentation proved by the SAAC Shelby American World Registry, this pre-production GT350 boasts a numbers-matching 289ci V8 engine, paired with the original four-speed T10 manual transmission.

And, according to the original Barrett-Jackson ad, this particular Shelby facets a rich history. For starters, this Shelby American test car was used for the driving tests for the 1966 GT350 development program. Curiously enough, it was also used to evaluate a vinyl roof treatment; one which we know was never released for sale as an option on any Shelby of that era.

If you were a dealer back in the early ’60s, then you might even remember this Shelby Mustang appearing brochures. Moreover, unlike the regular production GT350s, this Shelby was not a “delete” car from Ford that was later upgraded to GT350 spec by Shelby American.

“It began life as a factory-standard, Wimbledon White K-code high-performance 1965 Ford Mustang Fastback, with all factory-standard features remaining with this car,” say Barrett-Jackson.

Allegedly, this car (SFM6S001) was also the only GT350 ever to have been equipped with an upscale factory-optional Pony upholstery from the factory, along with the other production Mustangs. It’s quite interesting to see what did and did not make it to the production version of the ’66 Shelby GT350.

For example, this prototype Shelby featured aluminum inside rear-quarter window trim, and fittings for the rear-brake cooling ductwork.

“Late in its Shelby American days – during the last two or three weeks there – SFM6S001 was sent to ACME Auto Headlining in Long Beach, California, and fitted with a Medium Blue vinyl top, which was briefly considered as a potential factory-available option for 1966,” according to Barrett-Jackson.

We don’t think we could have written that note any better, though we’d be cheating ourselves if we didn’t admit that we are curious why the vinyl top never made it to production?

There’s a wealth of more information and history on the world’s only pre-production, prototype 1966 Shelby GT350 on Barrett-Jackson’s website. You can view the original listing for the car here before it sells for more than six figures come next year.