This is a guess, mind you, but I think it's a close-up lens (the center portion) with corresponding magnifier (the outer) for the viewfinder. Made like this for a lens with rotating front.
Location: Lungsod Quezon, PILIPINAS/Philippines/Филиппины
Posts: 2,200
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.
Jay
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Close up attachment for RF cameras, Proximeter I or II, made by Schneider. The RF correcting annular disc does not reach the Leica RF windows but they do fit a Retina and similar.
From Jason Schneider's On Camera Collecting, Wallace-Homestead Book Co., 1978
Cheers,
Abbazz
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