The FMX is similar to the older Borg-Warner (BW) M-8 and M-10. The M-11 and M-12 are improved models that have a totally different valve body. All the M-x and M-1x series of BW auto trannies have a cast iron center section. The BW M-3x and M-4x series are lighter duty with aluminum center sections.
Detroit Gear (DG) developed an auto trans in the late 40s along with Studebaker. DG was owned by BW, or at least was by the early 50s. By 57 the trans no longer had DG on it, just BW (57 is when AMC started using it). As Stude developed financial difficulties in the early 50s Ford stepped in to help finance development of the auto trans. Ford got licensing rights to build their own version -- the various "O-Matic" models (stick Ford, Cruise, or Merc in front) and finally the FMX. These are all similar the the M-8 and M-10, nothing like the later BWs. Many internal parts interchange between the M-8/10 and Ford models, some even interchange with the M-11/12 (mainly clutches). Ford cast their own case and valve body though, at least for the FMX and late model "O-Matics"
All the M-8/10 and "O-Matic" models are THREE SPEEDS. They follow a different way of thinking than later three speed autos (FMX is similar to later models). They normally start off in SECOND gear, with first only used for pulling heavy loads or rapid take-offs, which were discouraged, of course. On the BW models the shift quadrant reads P R N 2D1 L. The "2" and "1" on either side of the D are small and to the upper left and right of the D. In D2, which one used to later models would assume was "Drive" (shifting all three gears), the vehicle starts off in SECOND. Many assume these are therefore two speed autos. Put it in D1, however, and it will start in FIRST. They don't hold first very long, it's a real quick shift at 15-25 mph (depending on how fast the vehicle accelerates).
The Ford "O-Matics" work in a similar way, but I'm not as familiar with them. At least later models (but prior to the FMX) will start in first in what is considered the "normal" drive position. Earlier models, up to about 65, start in second and have the center forward gear position as a first gear start.