Vintage Vacuum Tube?
I have a vacuum tube from my collection here that I would like to test. It says on the box International, Electronic tube, GTE, Sylvania, And what appears to be 12Z - the last letter or number I don't know its either a 1 or a capital i, 12Z - I or 12Z - 1. It has 12 pins, 3 of which are not connected to anything, I can see that visually. Any help would be appreciated.