The connector on the back is for DC voltage input. It is not shown on the schematic but I saw an advertisement somewhere that mentioned it. Check out this page for info on the model 20A that is very similar:
http://uv201.com/Radio_Pages/emerson_20.htm
Jim,
That is a really neat set. I bought one about a year ago but it is not as nice as yours. Mine is missing the knobs and the back cover. I restuffed the capacitors and put a new power cord on it. I'm not in the best place for receiving stations on a 4 tube set like this so it sits on top of the kitchen cabinet. I love the wood inlay.