Pezon michel identification and dating
The shape of the cocks and bridges is more of an aesthetic consideration; so long as all the pivot holes and screw holes are in the same places, then bridges of very different shapes can be freely interchanged.
Some manufacturers produced many different movements with the same basic layout and identical train and keyless work components, but with different patterns of cocks and bridges.
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 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 keyless work under the dial that handles stem winding and setting is often said to be the "fingerprint" of a movement.
In fact, the apparently huge variety of Swiss watches is explained by this phenomenon: once the basic layout in a round movement of the barrel, train wheels, escapement and balance was arrived at, there was little scope, let alone need, to change it.
Compare the bridge shapes shown in Jobin for the Eterna movements and those seen on Stauffer branded Eterna movements.