Your imagination, probably, or else you're looking at different types of Pullmors. There were two major kinds of Pullmor motors, commonly referred to as Type I and Type II, in use from the Postwar period through the MPC era into the 1990s. Even within those, there are some variations. There were other types of Pullmors in use in some engines during the very early Postwar period as well, before 1954, and some which were only found in certain engines made in the late 1940s.
The Type I is a separate motor and truck assembly, and the motor consists of a single - or double - wound field, depending on the E-unit type, surrounding an armature with a worm gear. The worm gear fits down into the top of the power truck, where it drives either a gear on the main drive axle, or internal spur gearing in the truck which transmits power to more then one geared axle. The motor is mounted to the trick via a single mounting screw and pivots in a keyhole-shaped opening in the frame. This drive system is a descendant of the system Lionel developed in 1954, and was used in MPC and later times in Lionel's "premium" diesels and electrics, including the F-3s, FMs, EP-5s, and GG-1s. Postwar Lionel used this motor on its Geeps as well, but MPC used the less expensive Type II for the majority, though not all, of its Geeps.
The Type II Pullmor motor is known as the integral truck and motor assembly (intended to differentiate it from the Type I). This was the most common motor in the Lionel product line in the MPC era, other than their "premium" engines. The Type II was originally developed during Lionel's downsizing period in 1955 in response to cost-cutting demands. In this system the motor field, drive gears and axle mounting frame (i.e., the main body of the truck) are all one piece. The spur gears are mounted on one side of this piece, driving geared wheels. This drive system rides in a large hole in the frame, suspended from spring-loaded bushings traveling in curved slots fore and aft of the large hole.
This information is from Greenberg's Guide, and refers to diesel engines. Pullmor engines used in Lionel's steam engines is a completely different subject. Many of the motors used by MPC in their steamers were pretty much the same as were used by Postwar Lionel.
It should be clear by now, based on this, and John Korling's comments about F-3s, that there were many crossovers between Postwar and MPC era Lionel. There was no big gate that dropped and all of a sudden everything changed with MPC. Among other things, many of the "cost-cutting" measures many folks associate with MPC were actually initiated by Lionel in the Postwar period.
