What about the square batteries that you sometimes find in old cameras? Are they the same 4.5 volt batteries Bunsen mentioned or are they different?
Generic PRAM batteries
That all depends. There are many kinds of square batteries for cameras. I'm sure some are 4.5 volts.
The batteries arrived today and I put one in the PM 7500 that I am using right now. Seems OK so far. Rebooted several times already without losing the time and date.