Second Horn for 71 Demon

When a car is built with a single horn, is it "High" or "Low"?

My '71 Demon came from the factory with one horn. I would like to add a second horn, but I don't know whether I'd need the "High" or "Low" to compliment what I've got. (To be clear - when I say "compliment", I would want to add the "opposite", not another of the same)

Also, I would like an OEM horn, not a reproduction, and would prefer an original mounting bolt, too.

Thanks,
Michael Anzalone