It looks like a transformer for the dial light of a "Princess" model telephone.

Could be. What are the colors of the two wires?

Red is the wire sticking up, and then there is black and green which looks like the only 2 connected.

Darn, I was hoping it had something to do with our outdoor lamps that we can't figure out how to light up.

It may be a phone jack, whats on the side we can't see?

You mean on the other side of the plug? It says Trans 2012 Western Electric and then some voltage info. About a foot away is a phone jack. But why wire it in to an outlet and not just plug it in? I don't get it.

I am reasonably certain that it was for the light on the Princess phone, as already stated. They wanted to conceal the wire. Remove the wall plate, cut the wire so that the plate will conceal the remaining stub and reinstall the plate. That thing probably dates from the 1960s.

Here is a link that might be useful: Princess

The princess phones used black and yellow for power to the light in the phone.

Normal phone wiring is red & green.

The transformers all had recalls against them as a fire hazard.

No they did not "ALL" have recalls. The ones that were the issue were manufactured by the Ault company.

I have one of those and it looks just like that. The one I have keeps the battery to my alarm system charged. I have a Brinks box on the opposite wall with a dry cell in it.

When I moved into my hope the home owners had a setup like this. It was the power brick for the security system. They had the power cable go down through the walls from the control panel, then up and out to an outlet like in your picture.

Even crazier in my case - it was connected to a switch controlled outlet.