It's 2 wires. If you want to feel warm and fuzzy, try page 2 of the pdf
http://wedophones.com/TheBellSystem/pdf/bsp/2500/502-523-402-8205.pdf
A bell in a 2554 set has a red and black wire. You can't connect them directly across the line, because it would short out the line. The network has a capacitor between A and K, so you wire the bell through those leads for bridged ringing.
Now, some bells have the capacitor already on the side...ITT mini ringers do not, but the 75-BA's do.
Carl
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