If you examine an Übertrager, you will notice that the switching terminals (Schaltklemmen) on the primary and secondary coils have tabs. The tabs run between terminals AP2 and EP1 on the primary coil, and ES1 and AS2 on the secondary coil.
When the terminal thumb nuts are loosened, the tabs can be swung out, creating gaps in the middle of the primary and secondary coils. But, why do these tabs exist?
Well, for example, the Übertrager can be used as a makeshift replacement for the Amtsanschließer 33, if none are available to connect an Ortsbatterie Betrieb (OB) field telephone, such as an FF33, to an Schlusszeichen Betrieb (SB) exchange/central office (Amt).
The primary side terminals AP1 and EP2 are connected to the SB line from the exchange/central office. The tab between terminals EP1 and AP2 is swung out of the way. A 2 microfarad (2 µF) capacitor is placed between terminals EP1 and AP2.
The secondary terminals AS1 and ES2 are connected to, for example, a free pair of terminals La and Lb on a small, 10-line switchboard (Klappenschrank). In this example, to the terminals for line 1.
Now, suppose a call comes in from the SB exchange/central office on line 1. The operator presses in the tab on the Übertrager to short out the capacitor. The caller tells the operator that he wants to talk to the OB field telephone on line 10. The switchboard operator connects lines 1 and 10 with a patch cord (Schnurpaar) and the conversation begins. After the conversation has ended, the tab is swung out, putting the capacitor back in the circuit. The capacitor then sends a pulse to the SB exchange/central office, signaling that the call has ended and the line is ready to receive another call.
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WW2 German Field Telephone Equipment 