I recently purchased the polarizer for the 50mm Distagon C (Hasselblad part 50482, I believe), and am puzzled by the middle ring.

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It rotates freely between the filter and the mount ring, and doesn't seem to carry anything with it. The polarizer functions fine, but what is the purpose of this ring? It seems obviously intended to move, with the small metal "handle" (visible under the "B"). Anyone use this and can shed some light? Neither Wildi nor Google are helping...