JB,
Here's a site dedicated to the zenith TO radios. Looks like this guy is selling them, but wouldn't say that his prices are market value. It does show photos and years, and looks like yours is from 1979.
http://www.grandcanyontuberadio.com/...nsoceanic1.htm
There's lots of info on the web concerning these sets. I'm sure you'll get a good idea of the value by cruising the Google or Yahoo search engines. Looking them up on eBay might give you an idea of what it's worth. Regardless of where you search, you'll probably find a wide variety of price ranges.