DK-5 is used on the EM, FG, FG-20, N2000, N2020, N4004/5005 series, the N6000/6006 cameras, N/F50, 55, 60, 65, 70, 75, and 80, as well as many D-word Nikons. The metal eyepiece ring looks like it may've been from a DL-1 Photomic Illuminator. The diopter will work on any Nikon with round eyepieces from the F through the F3, the FM/FE/FA series, and the Nikkormats. Won't work on an F3HP, F4, F5, F6, F100, or the round eyepiece N8008/N90 series cameras without the DK-18 adapter ring. Also can be used on the aforementioned rectangular eyepiece cameras with a DK-22 eyepiece adapter, which also fits Pentax and Minolta cameras.
If the diopter were a -5, I'd be very interested. Ralph Javins might be interested in it, even though he's a +3. He seems to be taking a vacation from the forum right now, but might respond to a PM.
-J