James,
While your lantern is round, it will probably hold a standard, square 6V lantern battery just fine. If you look inside the lantern, you will see where the lantern has a center contact for the center post and probably has a circular contact which will contact the outer post of the square lantern battery no matter how it is put into the lantern. Give it a shot. You will be surprised. And please tell me if I am wrong. That happens on occasion, but I have put many Adlake electric brakeman's lanterns into service at our museum with square batteries.
As for the switch over from kerosene to battery, I don't know of any real firm date, but I would believe the transition was well underway in the late 1950's and early 1960's. In Richard Barrett's lantern book, he states that battery lanterns came into being around 1918 and became more popular after the depression.
I do know that even with battery lanterns well established, the Penn Central, for example, required cabooses to carry kerosene lanterns for some time after PC's formation. This is why you will find a lot of Adlake Kero short globe lanterns marked for the Penn Central. Our Penn Central caboose at our museum is equipped with several and a Penn Central marked kerosene filler can. All of them look almost new as they were rarely, if ever used in service.
Hope that helps.
Until later,
Chris
R&GV Railroad Museum
crhauf@frontiernet.net