With the recent introduction of the BMW M4 CSL, the tally of special vehicles from the BMW M department bearing the hallowed CSL badge grows to three. But as with many automakers, there are interesting prototypes that remain as internal projects, never to be seen by the public.

Fortunately, the marque has decided to reveal several of its one-off test vehicles as part of its 50th-year celebrations, showing off what could have been for the exclusive performance badge.

BMW M3 CSL V8 (E46)

You may be asking yourself: “Wait, doesn’t the E46 M3 have a CSL variant?” Yes, it does, but this specific one-off prototype has a V8 under the hood. Originally a press car, this M3 CSL was donated to BMW M after the launch, and it was fitted with a high-revving 4.0-liter V8 (known as the S65VB40) with 430hp. This car would remain a prototype, but the development and the knowledge gained would eventually make it to the S85 V10 and S65 V8 engines, the latter of which would make it to the E90 M3.

BMW M5 CSL V10 (E60)

The E60-generation M5 was famous for its screaming 5.0-liter, naturally aspirated V10 called the S85, making it one crazy executive saloon that many fondly look back at. But what if it had more power and less weight? Enter the M5 CSL prototype. While it may look like the sedan with a special M tricolor livery, forged 18-inch wheels, and an additional grille for an oil cooler, things were a lot different under the sheet metal. As part of the CSL treatment, it shed the rear seats and gained Recaro front seats and a carbon roof to shave off 150kg. While we don’t know the exact engine code for the prototype V10 engine, two were made: a 5.7-liter engine and a 5.5-liter version with 630hp and a high 8,750rpm. The sequential manual gearbox was swapped out for a dual-clutch transmission, and the suspension was fine-tuned for better handling. While this car never made it to production, it did lap the Nurburgring in 7:50, which was no small feat at the time.

BMW M6 CSL V10 (E63)

While it may look similar to its non-CSL counterpart, this specific M6 was BMW’s test bed for active aerodynamics. Aside from the weight reduction, it had an automatic rear spoiler, a retractable front spoiler, and the “double-strut” M mirrors, which are seen in today’s high-performance Bimmers.

BMW M2 CSL (F87)

“But the M2 CS exists!” Yes, the ‘baby’ M car has its fair share of hardcore variants, but did you know that we could’ve gotten this instead of the CS? The M2 CSL prototype takes inspiration from the track-focused M4 GTS, featuring elements like bucket seats, a rollover bar, a center console made out of carbon-fiber-reinforced plastic (CFRP), a carbon rear wing, and carbon ceramic brakes. With 450hp, 550Nm and reduced weight, this little coupe was no joke. Unfortunately (or fortunately), as the M2 CS had the same power output with more equipment suitable for daily driving, the choice was obvious for what would be the sportiest variant of the M2.