In order particularly to put a watch into beat and adjust the isochronism, in addition to the poising screws (2a, 2b) of the balance (1) and the fine adjusting screw (7) pressing on the index tail 20a to alter the orientation of the index (20), the device includes, on the one hand, two screws (19, 29) respectively fixed in the balance cock (10) and in the index (20) to maneuver clamps by means of L-shaped rocking elements (13, 23) to fix the end of the balance spring and define its useful length, and on the other hand, a screw (9) for immobilizing the index (20) in a determined position, these three screws (9, 19, 29) enabling a fine adjustment to be carried out.
