No calibration certificate or original inv. with specs.
Info on glass:
• The size of the component is 20.9” diameter x 1.250”.
As per my knowledge Glass description is;
• Window of aerial photography per MIL W-1366F, Group M
• Material: S-BSL-7/A5
• Finished Diameter: 530.50mm
• Finished Parallelism: <3 Sec
• Bevels: 3.00 mm max
• RYN-C Coating
The glass is still available- was a spare and never used.