The compartment on the side of the case hold some sort of a sensing component. When I unscrewed the back cover I saw some electronic parts but no battery nor a battery seat. I will let my watchmaker take a "deeper" look.
Yes - although I realize from your photo I'm not the intended target of your inquiry. That's a really interesting specimen you've got there! Love it!
Mine's an automatic, but since MG rarely gets discussed on OF, I thought I'd chime in. Since 2007, I've owned a M2 (N.O.S. from 1999). I hope you and your watchmaker are successful!