8 inch diameter precision optical flat that is aluminized and overcoated. It's from a DOD optical lab and has been in sealed storage. Appears to be a truly high quality flat due to it's source, its overall high-quality appearance, and writing on the back that says "This was certified" and "Tested OK". Should be at least accurate to one-tenth wave, maybe even better.
It is in very good condition - the only imperfections I can see are about 3 small (0.5mm) spots where the aluminum coating has been marred, quite near the edge. If absolutely perfect performance is required, re-coating might be called for. It is 1.2" thick and 8.25" in overall diameter (a .125" indention surrounds the optical surface). It appears to be made of Pyrex.
Since new mirrors like this are approx. $1300, I feel $450 is a reasonable price. Free shipping in continental US. I'll try to answer any questions you might have, but I've given all the info I can think of. I also have a large used 1/8-wave diagonal mirror (4.25" m.a.) that might be of interest (<$200).