Maker: Wittnauer (Showa-Koden)
Cell type: Selenium
Measure type: Reflecting/Averaging
This appears to have been made by Showa-Koden in Japan under the Unittic name; Wittnauer branded it in the USA as an accessory for their movie cameras. There's a version for Yashica cameras as well.
Some high-end 8mm cameras allowed interchangable lenses via the "D" mount, a screw-mount that went into the lens turret. This meter looks just like a lens, even down to the lens cap and focus rings. You screw it into the turret and it's always there, looking at wherever you're pointing the camera. It's selenium so you get a live reading all the time. You turn the ring to set the exposure index and then direct-read the ƒ/stop.
If you want all your turret places for actual lenses, there's a screw-on cap for the rear with a loop. Clip it on a lanyard and you can just hang it around your neck.
In the US, Wittnauer is most famous as a watchmaker. This meter comes in a nice clamshell case with a blue velvet pad just like a nice watch.