Typically systems of that age use series AC signal circuits. The adjustable resistors to set the current point in that direction. In these circuits the bells, horns, or whatever are a low voltage AC at some current rating. Look at a signal device data plate. If it says something 6 Volts AC 1.8 Amps it probably is a series AC circuit. These circuits do not use an end of line device. Rather, if complete, the circuit activates a gong circuit supervisory relay in the panel.
There may be only one relay for all the signal circuits. That is how an old Simplex 4247 system is set up. In supervisory mode the signal operating relay contacts are arranged so that all the resistors and all the signal devices form one big series circuit. If all is well the supervisory relay will activate. I don’t know if the Ellenco is set up that way or uses a separate supervisory relay for each circuit.