Soon to be auctioned: Cartier Tank Cintrée platinum.
Credit pictures: Monaco Legend Group.
According to them, it is a piece unique in platinum. Reference 2843J.
The only other Tank Cintrée I know is this one, belonging to our dear Geo:
As you can see, they are very close. The numbers are different, of course, as well as the cabochon in the top of the crown.