I bought a DK-17M. Why didn't they make it a couple mm larger in diameter? I hated it.
I put it on my Lensometer, (Old AO.) There was 3 diopters of prism. (The ocular center of the eyepiece wasn't even close.) Basically, Nikon Ground or molded a +0.75 lens the size of a Frisbee & cut 100 eyepiece lenses, "cookie cutter" style out of the implied 12" lens.
I phoned Nikon direct, explained & their reply was, "I don't care."