This mount adapter lets you mount an older Arri Standard mount or Arri Bayonet mount lens on a PL mount camera. This is the much more complex type with two spring-loaded push buttons for release, and is compatible with lenses like old Cooke Speed Panchros in Arri Standard mount. Those lenses need the holding tab that this adapter has to let you focus. Very easy to pop lenses in and out by just pressing two the side buttons.
Les Bosher was known for making these, although I'm not sure who made this one. I bought this one from Visual Products about 9 years ago. Duclos sells something similar new for $700: https://www.duclosle...dard-pl-adapter
Note, this works fine on most film cameras. I used it on my Arri 3C before I sold that last year. It may not be compatible with all PL mount digital cameras like the Alexa - the protruding back can collide with interior baffles.
Located in Brooklyn, NY. Thanks for looking.