The FM3A's "M" simply refers to the availability of the mechanical shutter which can be operated without electrical power at all (full stop) speeds. This puts it in the FM line of cameras.
If you open a Nikon FM3a and a Nikon FE2, top and bottom covers, what I have said will instantly make sense. Even holding the FM3a, FE2 and FM2n and observing the location of the controls will clearly show that the FM3a is not based on the FM2 but rather on the FE2.
The FE2 and FM2n may look similar from the outside but internally they are quite different. Again open the top and bottom covers of an FM3a and FM2n and the marked dissimilarity will be apparent.
Note that the FM2n did not offer TTL flash while the FE2 had TTL flash which was the reason I bought two FE2(s). The TTL flash sensors are located in the bottom of the FE2 and FM3a mirror box.
Thank you for posting the article of the design history of the FM3a. I read it complete. It is interesting to note that an ammeter was obtained from a used FE2 and used as the base for the ammeter used in the FM3a. The differences between the FE2 and FM3a are all about shoe-horning the hybrid shutter and controls into the FE2 design.
Dave