Just as an FYI and future reference. This information can also be found in the (just updated)
"Overview of the M System" article
here.
When replacing batteries in the M6, M6 TTL, M7 or the later MP bodies, the preferred replacement is a
single (two for the M7)
CR1/3N 3v lithium battery. They're not that common, so if you have any trouble finding one while out and about and in a bind, you can use
two (four for the M7)
PX76/SR44 1.5v silver oxide batteries. Do go with the better silver oxide batteries over the alkaline version if at all possible.
CR1/3N also known as/similar to:DL1/3N, K58L and 2L76 (11.6 x 10.8mm)
PX76/SR44 also known as/similar to:A76, S76, D357, 10L14, EPX76, KS76,SR44, 357, and V76PX (11.6 x 5.4mm)
The CR1/3N (or equivalent) looks something like this: