My guess is that it is a 2032 button cell - which have been popular for that function for a long time. Are you able to apply about 3V at the appropriate polarity and see if the memory comes back? Or even 1.5V as proof-of-concept?

After which comes the question of whether it is rechargeable (LIR2032) or not (CR2032).

Peter Wieck
Melrose Park, PA