The M1B Prototype was Bruce’s the official team car for 1965 season of sports car racing and although it shared a lot of similarities to the previous year’s M1A, including the same wheelbase, overall the bodywork was over a foot shorter. The all-alloy body was designed by artist Michael Turner followed Bruce's guidelines and featured in a blunter nose and sharply truncated tail with a tall rear spoiler.
Powered was delivered by a Traco-built 4.5-litre Oldsmobile V8 driving a four-speed Hewland LG gearbox with braking duties carried out by dual-circuit Girling brakes.
The car debuted in Bruces hands at the Canadian GP at Mosport taking pole and leading for 96 of the 100 race laps eventual finishing 2nd. It was next raced by Phill Hill at the Kent circuit in Washington State where again after leading for most of the race, a sticking throttle eventually finishing 5th. After a third outing at the Riverside Circuit, where Bruce finished 3rd, the M1B’s first victory came at the 1965 Nassau Speed Week where Bruce won the Governors Trophy race.
For the start of the 1966 season the Oldsmobile was replaced by a small block Chevrolet that offered an additional 100BHP over the previous engine. To offset the additional 200lbs weight penalty, a lighter ZF gearbox was fitted, and the chassis was modified with additional structural tubing to support the heavier drivetrain.
The car was sold by McLaren in 1967 to KarKraft, Fords development department in in Detroit, to be used as an engine test chassis for Fords IndyCar engine program.
In 1970 the chassis and body were rescued from KarKraft by Tony Antonelli who set about restoring and converting the car to road use which was completed by 1972, it remained with the family up until 2022.