Canon loved to introduce a very new camera which had a lot of cutting-edge features, and then sell a simpler, stripped-down version of it for people who didn't really want to go all the way. A perfect example is Canon's FX and FP, which are essentially the same camera; the essential difference is that the FX had a built-in meter and the FP did not. But you could buy this meter as an accessory that would fit onto the FP and give it on-board metering capability. Of course, if you were going to do that, why not buy the FX to begin with? That's probably why I almost never see FPs (or this meter) for sale.

This one sits more or less over and to the right of the pentaprism (if you're holding the camera as if you were going to use it). The shutter peed knob on the bottom right in the photo would fit over the shutter knob on the camera; so you turned on the meter (it's a CdS, so it takes a battery), select your shutter speed, and the meter would point to the aperture to use.