I agree with Fred appears to be the original magnifier, used for the K series and Spotmatics.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2915712799&category=15240 You also ask: "How does it attach to the camera? Does it take up the hot shoe?" No, it slips over the eyepiece, flipping down on a hinge when you need it. It does make the camera harder to fit in a bag, however. "Have you ever used one before, and do they really work?" I had one on all three of my SLRs. It truly makes focusing much easier, especially when your lens is a wide angle. Of course, when using the magnifier, you're looking at only the central portion of the image. I'd still be using them, but they kept knocking out my contact lens, and after losing a lens, I decided I couldn't afford to continue using the magnifiers, so I sold them.
