Bjarney you are right!! On the upper part of the duplex the slant pin is for the "long wire" antenna for old and/or short wave radios, and the vertical pin is the antenna system ground. These were available also as singles without the a/c power. We had one in my folks house. Since I have been a radio ham since youth, that connector fascinated me. They became obsolete when NFPA decided mains electricity and antenna connections don't mix, and can be hazardous to radios and human beings if wired backwards!
