WE 20AP candlestick question

[Witty]

I recently bought a WE 20PC and a 20AP candlestick phone. Both have the original wiring for receivers and subsets in the sets.
I'm familiar with the 20PC but can not find any information about the 20AP set. It has a small button on the stem near the top opposite the switch hook.

Is anyone on here familiar with this set, what system it was used on, and what the button is for?

I appreciate any information anyone can provide.

Thanks!