What is this socket?

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I recently moved, and the wall outlets in my new apartment include two standard three-pin electrical power sockets (two power blades plus a ground pin), which are used here in the United States and in some other countries. The wall outlets also have that mysterious four-pin socket. What is it? It took me a while, but I remembered. Do you know?

I’ll give you time to think.

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Hint: This building was constructed in 1973.

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Another hint: Ironically, this kind of socket, installed in several places in every room as a minor luxury, almost immediately became obsolete.

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Answer: It’s a 505A wall jack for a Bell telephone. This was where you plugged in your landline. In the 1970s, this kind of jack was replaced by the modular connector plug, which is still in use.

I graduated from high school in 1973, so I know about landlines. I remember the excitement when push-button touch-tone phones first came into use. (You could use the tones to play songs!) Still, I can’t figure out why I’d want a landline telephone now.