Debuting in the new Royal Oak Extra Thin, this caliber improves on its predecessor, the AP 2121, in a number of significant ways.

I am incredibly happy to see the barrel get a dedicated bridge. Previously, in the caliber 2121 (itself based on the Jaeger-LeCoultre 920), if there were any issues with the barrel, such as a broken mainspring, nearly the entire movement would have to be disassembled to address it. Mirroring it handsomely on the other side, I am pleased to see this caliber also make the move from a balance cock to a full balance bridge. The move from peripheral automatic winding to a centrally driven system, mounted on ball bearings, is a likewise welcome change.

I am curious to learn more about the specifics of the implementation of the new stud holder. Ostensibly, the mechanism to move the stud holder, in order to adjust the beat error, appears to be mildly over-engineered and the screw to lock in the beat error once corrected is precariously close to the soft metal of the balance bridge.