Friedland door bell and transformer

[SUNRAY]

Since always. Drawn one way, by the DC polarity, there is no spring, so it doesn't fly back, so you just get a ding. Drawn the other way, by the opposit polarity, it is drawn against a spring, which fires it back when the button is released, and so you get the dual ding-dong.

But this isn't a dual sounder unit.

 

[ericmark]

It uses 6 volt DC, but a higher voltage AC.

 

[Echo the husky]

But this isn't a dual sounder unit.

Yes the dual sounder one has some PVC wires connected to the terminals rather than just the 2 enamel ones in the photo.

 

[SUNRAY]

Yes the dual sounder one has some PVC wires connected to the terminals rather than just the 2 enamel ones in the photo.

I go by the position of the screw terminals but more importantly their "F" & "R" markings.
From the earlier link