Weird - the e24 was never an M6 over here but it appears it was in North America :-
What makes the M6535CSi/M6 unique?
The M635CSi and M6 are the BMW Motorsport-developed versions of the E24 6 Series. They use the M88 or S38 twin-cam 24-valve inline-six-cylinder engines first introduced in the M1. They also have a BMW Motorsport-tuned chassis and a few special cosmetic pieces.
What’s the difference between the M635CSi and M6?
All examples of the BMW Motorsport-developed E24 6 Series are known interanlly as the M635CSi. However, all North American versions were officially badged "M6", both to simplify the name and to bring it in line with the other M models, specifically the M3 and M5.