The filter on the card is a 600 Hz filter in the IF amplifier signal path, which is
switched in when using the two narrowest bandwidth options.
Both Telefunken and RFT filter designations use the letter E (= Einseitenband) for SSB filters, and
Z (= Zenter or "symmetrical") for a narrow filter.
I hope I'm not boring
The TD90 has revived, it has improved pushbuttons, I have replaced a battery, bad foil capacitors, it turns on and off nicely responds to everything.
Can it be connected to the receiver? Is it controlled by the receiver's frequency? Unfortunately, I don't understand your control without a drawing, I don't have any pin assignments for its connectors ...