Greetings,
I am the owner of a 38mm BP perpetual chronograph. There is absolutely no difference between the 2685 caliber and the 2585 caliber. They are identical.
Moreover, the new 40mm is not available with Arabics. A pity, I might add.
Below is a review of the watch. If it is not permitted it is on TZ under the watch review section.
I have had this piece for over 5 years, a marvel of design and it has been extremely robust.
Best,
MSB