Where'd they hide the battery in this camera?

Nikkormat EL, Nikon EL-W: It's inside the mirror box, behind the mirror! Remove the lens. Use the mirror lock-up control to raise the mirror. You'll see the battery box and an arrow showing which way to slide the cover. The battery is a PX-28.

Rollei 35 - When the camera is empty, remove the back. On the side where the film is loaded, there's a recess for the end of the film spool. It unthreads (a US nickel is the right size screw driver) and a mercury PX-625 should be inserted.

Rollei A-26 - Remove the end where the flash is attached.