Thanks. When I type in Philcotron in google I see many more examples. And like modern batteries, several cells are linked together in series to create enough power. Here's a screen cap of the radio full sized battery with ingenious +/- connector that a group of these would snap into. I guess the little 3rd opening was to top it off with the battery acid or water. Kind of cool, and now it goes back in the pile of valueless bottles, but at least I know what it is.