It's for a telephone.

Oh wow! Never seen anything like that!

Wow. A trip down memory lane.

https://en.wikipedia.org/wiki/Telephone_jack_and_plug

I'm working on a house now in a high end neighborhood- those jacks are (were) everywhere- at least two per room including the primary bath...

Here was another interesting relic from the house (at least I thought it was!) I'd never seen anything like it. :)

@Hallett - this house is in the high end historical area as well... do people... cover them? Replace them? Convert them? Just curious what I may hear with the walk through with contractor tomorrow (house shopping)

I think the era of hard wired anything is gone- even my tvs are wireless. Blank plate is fine, otherwise if you are having drywall work done just have them skimmed over and never think about them again

That cook center looks a lot like the one we had from the '70s, I think, and replaced during our reno in 2008-2012. Not sure ours was GE. Maybe Amana. More knobs. Same color.

kaseki (Hard wired - R - Us)

That GE Americana range is a high-end model for the time period. You can probably sell it if you're not planning to use it. The front-right Sensi-Temp burner has a temp sensor in the center that presses against bottom of the pan, can be set for degrees if it still works. The rectangular panel at center above the lower oven door is a viewing window. It's probably also self-cleaning. The angled panel above the cooktop is a light.

"do people... cover them? Replace them? Convert them?"

If you're planning to get a traditional telephone line (aka "land line") then you might use it. In that case, you could simply replace it with a modular jack (example) or you could get an four-prong-to-modular adapter (example). If you don't plan to get a land line, you can just ignore it.

In ours, instead of the pull out hood extension shown in the photo, there was a tilt up door that allowed direct (mesh filtered) air in to combine with that pulled through the microwave oven. Totally inadequate! Nearly totally inadequate even with a 600 CFM roof blower connected to the upper hood assembly via a duct and a transition. (Probably achieved -- sipped -- 100 CFM at best.)