Nikon Cross Coupled Meters
At the time of the introduction of the Nikon F, it was customary for photographers to use light meters for proper exposure of film; though many "serious photographers" actually considered such aids as an unnecessary crutch and preferred to make exposure estimates based on experience (lots of very over and under exposed pictures)
These hand held devices gave, employing proper skills, extremely accurate and useful data. The original hand held ones awkwardly required juggling of the camera and meter while proper exposure was achieved. The first evolutionary improvement was the ability to "shoe mount" the meter in the flash/accessory shoe of the camera. The employment of the shoe for this use made sense since it would not be needed for a flash when "available light" was to be exclusively used.
The next obvious evolutionary improvement after shoe mounting the meter on the camera for making readings on which exposure settings were to be based was the "cross coupling" of the meter to assist in directly setting the camera for correct exposures.
Nikon devised three different basic devices (plus variations) for this set of functions. All three clipped onto the body by grabbing the nameplate and coupled to the shutter speed dial of the camera body and F-Stop ring of the mounted lens:
Model 1 Type 3 cross coupled clip on meter:

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Model 2 Cross Coupled Meter:

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Model 3 Cross Coupled Meter:

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Clip On Meter accessories:

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Slide on diffuser and Booster for types 1 and 3 Clip On Meters