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This was on the wall of a 1972 house.

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  • Welcome to Stack Exchange. You might want to take our tour so you know how the site functions. A clearer picture maybe from a different angle might help. Given what looks like vents around the circumference makes me think it might be an inlet or outlet for an air system of some sort. Does there appear to be a vent behind it?
    – HoneyDo
    Mar 10 '20 at 23:03
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Although it is a really bad photo , I believe it is a 4 wire phone jack. These were very popular back then you could move a phone room to room at a time when each handset cost extra on the bill. To verify this normally small solid copper wires were connected red, green, yellow black insulation. On higher end homes there may be a 25 pair trunk instead the 2 pair cable , these larger trunk wires were normally used for intercoms.

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    Those were heavy duty plugs and the bells took 16V+- to ring. Then they went to those small clip in's.+
    – JACK
    Mar 10 '20 at 23:28
  • I did not remember the ringer voltage (I thought it was higher) but they were fairly large , the index line on top and if I remember one set of pins was closer together is what I remember. heck I probably have some males in my shop if the boys didn’t toss them when we moved here.
    – Ed Beal
    Mar 11 '20 at 0:55
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    I think you're right about the voltage being higher... the 16V was the wall wart for the light on the first princess phone. My mom had one.
    – JACK
    Mar 11 '20 at 1:05
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    Old phone systems used much more current that modern systems. Here's a photo of a surface mount jack: oldphoneshop.com/products/4-prong-jack-for-4-prong-plug.html
    – DaveM
    Mar 11 '20 at 2:37
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    Right, the voice circuit and the ringing voltage are different. While a single phone may have a REN of 1, the Central Office expecst multiple sets in a house, so supplying ringing for 5 REN would be typical. Poking around on the net, the ring signal would be upto 110v 20 cycle AC P-P on top of the 48v DC. If you plug in the 1REN impedence of 7000 ohms you get about 20 ma. Once the phone is detected to go off-hook, that signal is removed. Anyone that's been touching tip and ring when the phone rings, knows what I mean.
    – DaveM
    Mar 12 '20 at 3:56

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