For the first three decades of its life, Canon primarily made quality Leica-copy camera bodies. They looked similar to, and were compatible with, Leica M39 screw mount bodies. Like the Leicas, they did not have meters physically designed into them; selenium meters generally needed large cells—small cells didn't generate enough electricity to be much good. So you could either use a hand-held meter, or you could get one that clips into the accessory-shoe, as this one does.

This is similar to the Canon FP and my Yashica, in that they were designed for a specific camera so that the shutter dial on the meter covers and controls the shutter dial of the camera, so you have a direct linkage between the meter and the camera. But because it's modular, you could disconnect it and use the camera without it.

I found this in the back of a brochure for the original Canonflex camera, where they advertise other Canon products. This clip-on works on both the VI-T and the P models, neither of which I yet own. Someday. . . .