Circle-T (or University Optics - same thing, just re-branded) Japan-made orthoscopic eyepiece 7mm focal length. These are colloquially known as "volcano tops".
I'm looking specifically for the 7mm, and only the 7mm.
They don't make 'em anymore, but there are probably a few of these around, buried in people's collections. So, if you don't use yours anymore ...
