Sears Penske dwell meter

[towbar]

So being that I have a couple vehicles that still use point style ignition timing, I decided to buy me a dwell meter so I could accurately adjust the point dwell timing. I made my purchase off ebay, the purchase arrived within a a few days and I was really pleased how good of a condition this vintage piece was!! I was very excited so I jotted down to the garage to try it out on my c10 and Nova....well unfortunately it doesn't work. Taking it apart I found an old durscell made by Mallory, 4.3v battery, which looks like a fat AA battery....well of course this 4.3v battery is no longer available. I can find a 3.7v and a 4.5 v replacement. so now I come to my friends and experts on which battery would work well with this meter...will a 4.5v fry it, will a 3.7v work just fine??

I know there are some battery geeks out here that are shaking their heads in disbelief of my question....but your help would be appreciated

thanks, Lenny