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Hi Stuart,
As Daniel says, 'circle K' stands for Kasai as the maker of the eyepiece. Japanese optics are hallmarked typically, but not exclusively, by the 'Circle ..." stamp. The hallmark is the maker's ID. Can also be a square shape, diamond, rhombus, even unframed. The design of the optics, be they Kellner, Orthoscopic, Plossl, etc, is indicated either as an abbreviation or written out in full. Much confusion surrounds some of these hallmarks, as some makes may have had different names, but the parent company may have been the same, or the company changed names, but the hall mark stayed the same.
The photo below is an example of 'Circle T' (Towa?) and the orthoscopic abbreviation. The second pic is of Takahashi Hi-Orthoscopics - triangular hallmark.
Some of these Japanese manufacturers are no longer in operation, but their eyepieces are highly sought after as their quality is second to none. Even the old Celestron ortho eyepieces that are Japanese made are damn good - real gems of eyepieces, yet they fly under the radar.
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