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The most significant part for identification is the largest part, the cover plate.This has an integrated detent spring which holds the setting lever in either the winding or setting position.The really definitive "fingerprint" of a movement is the keyless work, which is why materials parts suppliers such as Bestfit and Schwartchild often show only the components of the keyless work in their movement identifications.However, to see the keyless work requires the dial to be removed, which may not be possible, and many older movements are not documented in these references.By looking at trademarks on plates, apparently different movements with the same keyless work, and other sleuthing, some progress has been made. The image here from the "Official Catalogue of Swiss Watch Repair Parts" issued by the Watchmakers of Switzerland in 1948 shows how movements are measured for diameter D, which is the dimension that matters.Note that the diameter D is taken from the part of the movement immediately below the widest part of the bottom plate.The setting lever spring was an evolution of the earlier simple cover plate that simply kept all of the parts of the keyless work in place.
Note that the setting lever screw is an exception to the rule about screw placement, it can be in different places in stem set and pin set varieties of the same movement, or absent altogether if the keyless work is negative set.
The fingerprints in the Bestfit book can be used to easily identify these movements.
Movements from the 1920s or earlier, with cover plates that don't have integrated detent springs, don't generally appear in the Bestfit or similar books and are therefore harder to identify. There are hundreds, possibly thousands, of different Swiss movements.
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