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As I have been culling unused equipment to put on the marketplace, I came across this ring, which I cannot identify for the life of me. It is not a coupling ring, a lens adapter ring, a Cokin-related ring (as far as I can tell). Aside from the photos below, here's a description: black, plastic, looks to be 49mm threaded on the side that screws onto a lens (?) but does not appear to be threaded on the other end. It's almost as if it is designed for something to rest inside the other end. There are no identifying marks at all. I'm sure I knew at one time what this was....
Help! Does anyone know the name of this creature? I'd like to sell it, along with some other equipment but don't know how to list it.