A most unusual meter. The diameter of the face is about the size of an old Eisenhower (or Liberty) silver dollar on a 3-inch cylinder. The brass piece in the upper left is the battery chamber. There's a slide on-off switch on the side that's not visible. The discoloring on the face around the zero-adjust is the residue from some Scotch tape that I haven't been able to remove.

TTL (through the lens) metering didn't become common in cameras until the mid to late 1960s. Before that the meters were either mounted outside the lens or not at all. For a lot of things that was fine, but what about macro photography, where you're focusing on something close and you can't easily meter it?

So what you do is this. You set up the camera with the bellows or extension tubes and get everything ready. Then you remove the camera but leave the rest of the rig alone, and you screw this meter in its place. The back end of the meter is threaded so it'll screw in directly in place of the camera body. You make your meter reading, unscrew it and set it aside, then reattach the camera and you're good to go.

This Model P means Pratika/Pentax Universal Screw Mount (more modernly known as the M42 screw-mount), which means it'll fit a ton of cameras from the era (Pentax, Pratika, Mamiya-Sekor, Miranda, and Zenit all immediately come to mind). They also made a Model T that took T-mounts, which was a modular system that adapted the M42 to various bayonet-style mounts.