I didn't want to open mine, but then I remembered you can remove the hatch and the module will still remain powered up (I didn't want to have to reset it). It's got an AG12 in it.

Yeah - I remember how shocked I was when I discovered this on my 0634.
I thought maybe it was fluke - but that's way they made them.
Thanks for the info re: the battery. Someone told me to use a '301' which corresponds to the AG12. But someone else told me to use a SR43W. But I remember reading an older thread that recommended using alkaline batteries for LCD's and sivler oxide for LED's.