ROBBIES
ELECTRONICS EMPORIUM
Here is a Telephoto Camera Lens Century tele athenar 300mm lens for movie camera Arri positive lock (PL) mount
These are optimized for movie use. Lenses like this are quite simple in construction. A well-corrected achromatic doublet with an optical flat in the front and rear. These are razor-sharp optics and take amazingly good movie film.
Century Precision Optics is still in business. It was acquired some years ago by Schneider. It continues to cater to the higher-end needs of the movie industry, which has always been one of its bread-and-butter applications.
From an ebayer with more knowledge than I " I suspect the lens pre-dates the Arri introduction of the PL mount. Looks like an Arri Standard mount which preceded the Arri Bayonet and the Arri PL."