The "1" in CAR5A51 signifies steel, which with these watches usually means a steel centre case, with Gold sides and lugs. Never knew that the standard model had elements of steel in the case. "4" denotes Gold...so must be a pure Gold case. Quite why you'd go and make two versions of an almost identical watch that are very, very low volume any instance I don't know!
@Calibre11 - I understood that the 3rd character after the letters (the 5 highlighted above) is the case material, where:
1 = Steel;
2 = Two-tone;
4 = 18k Gold;
5 = 18k Gold / Steel;
8 = Titanium; and
9 = CMC or Ceramic.
The 1st character is the type of movement (5 = auto chronometer);
The 2nd is size (A = Magnum); and
The 4th is the dial colour/finish.
I think there are two different watches in that info found by Hubert:
I would say the picture on the left is the “standard” MikrotourbillonS CAR5A51:
And the pictures on the right are from the “1 of 1” CAR5A54. It seems there’s one additional gold bezel on the 54, the power reserve indicator is also different, and even the tourbillons have some different parts: