This is the window for the left hand hatch of the Gemini IX capsule, which flew in space in June 1966. The window's outer glass assembly had a single pane and the inner glass assembly had two panes. All three panes in the right hand hatch window were optically ground for better resolution. Corning Glass made the panes of Vycor, which was 96% silica.
McDonnell Aircraft donated them to NASM in 1969.
Gift of McDonnell Aircraft