In the 1990s, the Mercedes-Benz lineup was far simpler than it is today. There was no AMG-GT or Maybach or SLR or SLS or coupe SUVs or hybrids. In fact, there was barely any AMG at all — AMG was little more than an independent tuner, like Brabus. This meant the SL was the top dog of the Mercedes lineup at the time.

The R129 SL, as car geeks call it, is often revered as the last old-school Mercedes. It was the most prestigious model the brand made before the merge with Chrysler. Many enthusiasts view the Silver Arrow special editions and the six-liter V12-powered SL600 as the ultimate versions of the last old-school Mercedes. But they weren’t.

This decade also saw the birth of Pagani. In 1999, the new brand showed off its first car, the Zonda, at the Geneva motor show. It was cool, but many wondered whether the Zonda would go down in history as a one-hit wonder.

Of course, it didn’t, and it’s now considered one of the greatest exotic cars of the time. And over time it evolved and became even greater. At some point, the AMG-built V12 grew to 7.3 liters. But this engine didn’t come out of thin air. AMG had built it before and stuffed it into a car.

This car was the SL73 AMG.

The SL73 started life as an SL600, but AMG decided a six-liter V12 was too small and ballooned the displacement to a whopping 7.3 liters. Power increased as well — to 515 horses, thirty more than the contemporary Ferrari 550 Maranello. And the torque figure was even greater: 553 foot-pounds. This propelled the SL73 on to a top speed of 186 mph.

Unfortunately, AMG was far from a volume manufacturer back then, and SL73 production makes the Zonda look like a Model T by comparison. While Pagani produced a total of 140 Zondas, AMG converted only about 85 examples of the SL73. And the Sultan of Brunei owns about 50 of them, meaning there are about 35 in circulation. This effectively makes the SL73 rarer than a Bugatti Divo.

Unsurprisingly, the SL73 does not come up for sale often, so it’s noteworthy when one does. And one is headed to auction at RM Sotheby’s later this week, finished in a gorgeous deep blue. It’s not perfect — the odometer indicates over 150,000 kilometers, and some wear and tear is visible here and there. But that’s part of what makes the SL73 so neat — you can drive it. It’s not terribly impractical and attracts zero attention unless you really know what you’re looking at.

What will this car sell for? It’s hard to say, since these pop up for sale so infrequently, but it’s reasonable to estimate one or two hundred grand. But given that the Zonda sells for millions, and that this car is even rarer, that seems like a bargain. The SL73 AMG is a personal automotive holy grail of mine, and I’m excited to see what it brings.