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.

Karen Nakamura over at Photoethnography says this was compatible with both the VI and the P. Since I don't own either one, I'll have to take her word for it. That's why I'm calling it an "RF" meter instead of a VI meter.