4 Inch Interferometer Flat 40QB20-6

Interferometer Flat 40QB20-6, 4 inch.. – Diameter: 4 in.. – Thickness: 1 in. – 1/20 wave – 2.2g/cm: is probably Fused Silica – S1: Uncoated, Smudges unsure if there is a small scratch S2: Uncoated, Needs light cleaning