Model: FP accessory meter
Cell type: CdS
Measure type: Reflecting/Averaging
Sensitivity Range: LV 1 to LV 18
Angle of measure: 40°
Battery: one 1.3V mercury cell
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: except that the FX had a built-in meter and the FP no meter at all. 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 meter is particularly interesting to me because it's the only one I've seen that actually covers the pentaprism and looks like it was made onto the camera. The first time I saw an FP with this meter installed, I thought the prism was detachable, the way Nikon and Miranda used to do it. Canon had interchangable prisms, but only on their professional-grade F-1 and A-1 cameras. Most camera makers had their accessory meters clip onto the accessory shoe, like Canon's own Booster, or the Yashica Penta J.
I've had this meter for years but only recently purchased an FP body. I don't know why I didn't do it a long time ago. My Canon rangefinder meter is unattached because their early rangefinders are so expensive, even today. But most Canon SLRs, even in good condition, are quite affordable for someone who's just going to set it on a shelf and admire it.
The instructions to use the meter are part of the full camera manual.