Enjoy it, but the batteries are under that side holder and they are no longer made as they were mercury cells, PX 13.
I'm no stranger to the PX13, as it was quite common in 60s and 70s SLRs-I have a lot of Canons that use it, along with my Nikkormat and my grandfather's Konica. I've always used a type 625 Zinc-Air hearing aide battery. These give the correct voltage(1.35V) and are stable over their lifetime(like Hg batteries), although the downside is that they only last a couple of months. They are cheap, though-about $10 for 8 and last ages in storage. I'm working out of a stash that I bought in 2008 or so, and they will come right up to 1.35V within a few minutes of pulling the plastic tab. There's a commercial product called the Wein cell that is a 625 in a metal ring to replicate the size and shape, but I usually just let a 625 rattle around in the chamber. For two battery systems(like the Photomic or the booster for my FT/Pellix), I generally wrap two in scotch tape and then poke some hols in the joint to allow air circulation.