I have a very similar one called "Pix Eye". I googled it and found a pic of one on the web:
http://vintage-electronics.com/photos/6740.jpg
I've used it 3 or 4 times and it does exactly what it says. Proves out your video and B+ and bias. Shows operation of brightness and contrast controls if your CRT is dead. It's a novelty to look at, but you can accomplish the same with an O'scope. Back in the day I guess the serviceman couldn't carry his O'scope with him. This thing, he could...