Yes, the LX (HX) (TWIN -TURBO) is the largest main case model of the three. The FX (CRUISE-O-MATIC) is the smaller case and the MX (MERC-O-MATIC MULTI-RANGE) is the intermediate case and is used in conjunction with the FX depending on HP and torque out put of the engine. All of these models are DUAL-RANGE (redesigned in 1958-previous design single range). The true F/M/2 (PX) is an economy two-speed (all others are actually three speed) transmission used from 1959 to 1964.
The design was again updated in 1968 combining the MX internals with the FX main case creating the FMX (DUAL RANGE-SELECT SHIFT).
The LX was replaced in 1966 with the C6.
Your LX (PCA) has a special output shaft to accept a special yoke for the driveshaft.
Here is a 1958 TSB outlining transmission allpications;
