I can't find a manual or any useful info on the net. I just have a simple question,
since I haven't used this thing in decades, of whether there is adequate impedence
or protection that I can just plug the two prongs into a wall outlet and read 60 Hz?

It's *very* unlikely that it will accept 115v input. Only connect it
directly to an AC outlet if you want to damage it, and possibly yourself!

The input range is more likely to be no more than a couple of volts. If
you really want to measure 60Hz AC, find yourself a "wall-wart"
transformer with no more than say 5v AC output, and try using that both
to isolate you and the meter from the mains, and to reduce the voltage.

The red and black connections are ground (black) and signal (red).
Nothing to do with positive and negative, or line and neutral.

Peter