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Are straight lines straight? That's the primary criteria on fisheyes - they have uncorrected barrel distortion. Looking at the pics, it seems like that might be the case, particularly near the edge of the frame. It's probably a full-frame fisheye, e.g. 180 degrees across the diagonal of the full-frame image. (like the canon EF 15mm).
Fisheyes, in general, do have low flare and good sharpness. It's simply a by-product of the optical design, partly due to the fact there's no barrel distortion correction. It also limits the cos4 falloff (much less falloff on fisheyes, in the corners of the frame.) vs. rectilinear lens design.
if there's an optical diagram of that lens online somewhere it would be easy to tell if it's a fisheye. I think it is.
Edit: after looking at the rokkorfiles page, indeed it basically is a fisheye.
He's right, the rear filter is part of the optical design - keep it in place. Without it there will be focus-shift forward (e.g. towards the lens), of some fractions of a mm, but enough to matter.