This may not belong on a car site, but it's Phil's. It looks like it is a high voltage probe for a meter. Phil was a Ham radio operator. This may be from test equipment he used to test “plate” voltage on his transmitters. I figure, due to the single creep-age flange, it is meant for above 1000 volts and below 5000 volts.
http://home.swbell.net/vkathode/high_amp_probe.jpg;
http://home.swbell.net/vkathode/high_amp_probe.jpg;