Location: Lungsod Quezon, PILIPINAS/Philippines/Филиппины
Does the glass outside the central disc look different from the central portion? I think it's an accessory for the wide-apeture 'portrait' teles whose softness depended on the aperture setting. When such lenses are stopped down, the softness decreases. An accessory like this is meant to retain the softness, at least reintroduce it, when the lens is stopped down. With the lens closed by as little as two stops from maximum, the aberrations which originate from the lens edges are lost and only the sharp central portion of the lens is used. The middle portion of the attachment will then restore the softening by introducing some aberrations of its own. Just a speculation.
不管黑猫白猫能抓到老鼠就是好猫。("Black Cat, White Cat, if it can catch mice, is a Good Cat"-Deng Xiaoping 邓小平 )