The A Michel 520 pocket watch movement is an open face caliber available in different options such as bimetallic screw balance and Breguet hairspring. This movement was produced in various sizes, including 18.5 and 19''', with heights ranging from 4.2 to 5.9mm. It features different bridge layouts, two base plates with varying dial-feet positions, and two slightly different setting-lever springs. While the assignment of these differences to specific groups is only partially known, comparing the corresponding calibers provides some insight. The A Michel 520 movement falls into the FHF 1 to FHF 16 groups, which encompass different dimensions, starting with FHF 22 for individual designations. This movement offers the functions of stem winding and setting, sub seconds, and was commonly used around 1940.