This is the Nikon O-7 which was made for the Nikon 10cm ED apochromat. These eyepieces are seldom seen in the West and difficult to find even in Japan, where they sell at several times the price of the Pentax smc orthos. These eyepieces (and the telescopes) are renown for the precision with which Nikon figured the lenses and the resulting definition achieved in planetary and lunar observing.
If you know what this is and want it, drop me a line before it disappears. Thanks for looking.