This is 4” diagonal secondary mirror that came as part of an optical set with my 20” f/5, Russian made, Astrositall primary. It has never been touched or mounted. I used a 3.1 inch secondary instead.
Do not know specs on this secondary except that I expect that it is of VERY high quality since the interferrogram for the associated primary indicates 0.987 strehl ratio and is a real performer in my Obsession scope.
$70 shipped CONUS from AZ