Post WWII Motors
A picture is worth a thousand words as Roland said:
Note: the picture shows an example of each motor type, not every variation.
The E20R motor was replaced with the E15R motor on the right. The E20R motor and the E15R motors are universal motors and can be run on AC and DC. The advantage of the E15R motor is that it can be run on a 12 volt DC, a car battery. When run on DC it was much quieter and more powerful. Apart from the internal differences if you looked at the front face you could see the difference stamped on the switch plate as shown in the image below:
Photographs courtesy of Roland Jaggard and Martin Hanson.