Gunter Österloh calls them doubling rings, part number is 14838. They seem to be quite thin on the ground.
I was after one myself and asked Malcolm Taylor if he had any. He sold me a pair of Sonia (the font they use for their log is difficult to read and I thought it read Sorria or even Somia, but a Google search seems to pick up on Sonia - they're made in India) rear lens caps instead, as he says that they are the best quality caps available (and they even have the correct part number stamped into them). I've yet to glue them together, back to back, as I'll have to sand off the lips to provide more purchase for the adhesive.
Nick