The Felsa 366 is a mechanical watch movement that was commonly used around 1935. It features a bimetallic screw balance and is part of a series of movements including the 865, 870, and 874, with differences that are not well known. It is also worth mentioning that the ETA 871, which is part of the same series, includes an indirect sweep second function. Additionally, the Felsa 366 movement is manually wound and includes a sub-second function.