A modicum of internet searching indicates that some tenders utilized an Eveready #750 three volt "pocket flashlight" battery (essentially 2AA cells in a case with specialized contacts).
Edit: The rival website shows a "Banner" F4 battery made by Burgess Battery in situ in a 120 tender. It appears to be the same construction and style as the above #750.
I think the post about the Banner F4 battery in a 120 tender was made by me. That seems to be the details that I recall about the odd battery I found. Took me a while, but I found the pictures in my Shutterfly account. Here is the tender as I found it
However, the C cell battery is likely correct for the smaller tender that Steve has. I recall finding a windup tin streamliner that had a C cell batter in it a number of years ago. Sadly that battery had leaked all over the engine.
NWL