This is an early Heathkit Isolation transformer model IT-1. Primarily designed for techs working on the old 5 tube AM radios of the 1950’s and other radios with a hot chassis.
It’s really a small variac for adjusting the line voltage – 100 watts continuous / 200 watts intermittent.
Working and in nice condition. I see these over priced on eBay for as much as $100. They are not worth $100. Although it can be used as intended, we are selling it as a collectors item for your Heathkit museum.
Here’s a full description; http://www.w6ze.org/Heathkit/Heathkit_018_IT1.pdf