The ref. 2531 is a rare one with a low production number among rectangular models powered by cal. 9-90: it is estimated that only around 200 pieces were made, of which approximately 2/3 were yellow gold and the rest were rose gold.
Honestly, I wasn't expecting it to fit and look so nicely on the wrist.
Have a nice week-end.
YELLOW GOLD
ROSE GOLD

credit: onbehalf