Geo
5188
It's not a Tank
But what it is exactly, I do not know yet.
Around 1974 Cartier released a series of different models where your watch belongs to.
The Vendome, the Square Incurve, the Gondole, the Cristallor, the Ellipse,, the Coussin and the Square wristwatch.
Yours comes close to the lines of the Square Wristwatch and the Christallor, but it is different.
To the Tank family it does not belong. If I do find the exact model, I'll come back to your topic.
GEO