The only way is too test the line. Hook up the antenna in the attic and then test the in question outlet to see if that makes a loop or use a 9v battery and test for the power.
> TEST the unknown outlet for power 1st for safety<
If the cable is from the 70's I would try to rewire it. Once you find out what it is, pull the outlet and look for any markers on the cable to try to date it and see if it is shielded and copper core.
My house was built in the mid-1970's and has a very large television antenna in the attic (guess they didn't want it to be seen outside?). In several locations there are small "outlets" on the wall that I am assuming are antenna connections. The ones in the upstairs portion of the house have a small plastic plug in them that can be removed to reveal two small metal nubs about 3mm in diameter and 6-8 mm long. The plugs read "Slater" on them.
Can anyone confirm that these are indeed connections to the antenna? If this is an antenna connection any ideas as to where I could obtain connectors so it can be used? The small indoor antenna that I am using along with the DTV box only gets a good signal from half of the stations in my area the rest are so-so.