There is the possibility of the motor portion being for 6V and not 12, especially if it was for a car radiotelephone. Some Genemotors were meant to be used with a carbon pile regulator. I have one such item in my collection. It's meant for a 24 (28V) supply with carbon pile regulator which drops the supply to 18V for which the input is designed. A carbon pile regulator is an electromechanical voltage regulator that works on the principle of carbon discs being pressed together at a greater or lesser pressure to control the resistance. What forces the discs together is a spring with a solenoid to offset the spring pressure. Increase the solenoid voltage and carbon pile resistance goes up.
So, the carbon pile is in series with the genemotor input with the solenoid across the genemotor input. If supply voltage rises, which it will when the output load is small, then more current into solenoid, carbon pile resistance increases and genemotor input drops. Simple and effective enough for the circuitry it powers. Voltages of 300 and 600 are certainly not unsual either. I use 24v or 18v genemotors on 12V and get a proportionally lower output which is more convenient for typical valve receiving applications.
There is indeed a blower attached to many units, and sometimes a channel switching mechanism in aircraft transceivers also coupled to the drive shaft.