Model: Penta-J accessory
Cell type: Selenium
Measure type: Reflecting/Averaging
In the early to mid-60s, many cameras still had no on-board metering, so camera companies began selling meters that could clip onto the accessory shoe. A few, however, went a step further and offered meters that integrated more directly into the camera itself. The Canonflex used one, so did the Minolta SR-1.
This one goes to a Yashica Penta J. The photo here shows the top. The selenium cell is pointing downward toward the ground. There's a shoe on the back, so what you'd do is clip this thing onto the camera, and the shutter speed knob (top of unit) would cover the shutter speed knob on the camera. The shutter knob is linked to the calculator dial and spins it. So you aim the camera and turn the shutter knob on the meter to the speed you want, and then the needle points to the proper ƒ/stop.