New acquisition. Runs properly on 1,2 V rechargeable long life Eneloop battery. 1,5 V dry cell battery was the cause of too big pendulum swings, with hitting of the coil. The dial may be replacement (unusual ATO lettering version).
Good photos of a historic ATO battery timepiece. It is odd and unusual to have an ATO clock swing the pendulum too much.
The brass ring damper should prevent excessive pendulum motion. The brass ring, a short-circuited one-turn electromagnet should, in theory generate a small current in the ring that creates an opposing magnetic field to repel the right end of the magnetic pendulum rod. The ring is "self-exciting."
If, Piotr, the single dry cell was located behind the pendulum and if the cell had a steel jacket, the steel may have disturbed the magnetic field of the pendulum motion but I cannot understand how that would cause excessive pendulum motion.
My Junghans-ATO master clock (in the photo below) has a similar pendulum and operates from the 1.5 volts of a single Alkaline dry cell. It too has a damper coil on the right but on mine, the damper ring is decorative.