This mirror is rather scarce, I think sigma only made it for a relatively short period. The
sigma history pages indicates two separate introductions, the first in 1983 and then again in 1985. There's no link to a specs page to either listing so I don't know precisely what the difference is, but trawling online listings I note a white version with a tripod mount and a black version without (mine).
I splashed out a bit on this one to satisfy my curiosity and to include it in my mega 400mm supertest (pending, but don't hold your breathe). PKM mount, a really nice minty one well worth the price in fact.
The light on the estuary on Monday was really exceptional, bright, clear, almost translucent. Mirrors especially like that (more accurately they don't like the converse, consider leaving at home if the light is poor), and this sort of subject.
My usual test pics of the castle showed that resolution tends to unexceptional, pretty good but struggling really to keep up with the best contemporary 400mm f5.6 refractive lenses IMO.
At the RSPB this was the best of a sequence of pics of a robin.
You know, I am still seeking the ultimate combo of lens, light, opportunity that will deliver a pic of the castle that makes me go yeah, 3D pop wow! Hasn't happened yet... these shots from the bird reserve across the estuary always seem to be a bit... I dunno, muddy? Lots of tinkering with contrast, blacks etc while processing this.
I'll take it out again with the tamron SP 350mm next (when the sun shines, might be a while)...