Summary: Years ago I bought a DG-2 magnifier and a DR-3 right angle finder for my F3 and my Nikkormat. For less than [$] I still can use them on my D300. What a deal. It's only plastic and may eventually crack but for that price I bought two.
Summary: This is a plastic piece which does work as promised. However, this is my second one to use with Nikon's DR-4 right angle viewfinder accessory. The weight of the right angle viewfinder is a little too much for the plastic adapter, which, over time and with use, eventually broke. Still, its an inexpensive way to mount a DR-4 right angle adapter (round mount) on a D70 eyepiece (rectangular mount).
Summary: I just love using the DK-3 rubber eyecup on my FM2n and thanks to this adapter I can use it on my D200 too! But beware, you will also need the "Nikon Replacement Eyepiece Finder for FE, FE2, FM, FM2, FM3 - Mfr# 2925" to attach the rubber eyecup to this adapter!!
Summary: Fits perfectly on my D7000. Allows the camera to slip on round eyecups and makes the viewfinder more comfortable when holding the camera in portrait. Round eyecup covers eye perfect and allows very little stray light in. Also gives the camera a very professional look and feel. Great purchase. Get it! then get a round rubber eye cup to go with it.
Summary: I have had a 'love affair' with the Nikon SLR system for over 40 years but even Nikon doesn't always get everything right! Here I'm thinking of their early ventures into auto-focus with the F501 body which was my first experience of a rectangular viewfinder. This came with a nasty piece of plastic (DK-5) which could be threaded onto the camera strap and was provided to blank off the rectangular eyepiece when necessary.