The ETA 2540 was the base model of the 2540 series of 7 3/4 ligne ladies' movements. It uses a direct driven center second, a moveable balance stud and an Incabloc shock protection system.
On the dial side, there's a little surprise: The hands setting wheel is beared in a jewel. This kind of construction is pretty unique, but technically not neccessiary, since that wheel runs very slow and is not within the force flow. Since the mainspring barrel on the ETA 2540 is not jewel-beared (on the dial side a 5 o'clock), this is one of the possibilities to reach the "magical" number of seventeen rubis.
The ETA 2540 is not much different to the later 2660 series, but one difference, which is important for the service is, that the setting lever on the 2540 is secured by a screw, while on the 2660 it is fixed by a spring. Another difference is the dial securing: On the 2540, two lateral screws were used, on the 2660, there were two quick-fixating mechanisms on the movement plate.