I'm an AC voltage guy by trade so DC voltage often times frightens and confuses me. However, looking at the schematics, I would think that the DF leg is more of a control voltage. If there is an issue with the motor, such as a cooling problem or clutch issues and amperage is running high, then the DF would blow, disconnecting the main power (supplied from the CB) at the switchboard. If there is a direct short or grounded winding in the motor or supply power, the CB would trip. By definition a circuit breaker is an amperage device. If the DF directly affected the CB, then the CB would actually be a relay or magnetic contactor that is directly commanded by another source.
I'm unfamiliar with 1940's German/European schematics so I'm not totally sure of the symbol used for the CB in Don's schematic. It appears to me to be a switch but it has a couple extra bits to it. In the section below from Tore, it looks like either a manual disconnect or magnetic contactor with overloads located on the load side of that switch.
From a modeling standpoint, I guess this is all moot but it is fun to try and decipher how all this worked. Would sure love to see inside one of those cabinets. Don, your last picture is about what I was expecting the components to look like.