We had these before and sold out. Just got more in. Manufacturer's part #02 MFG 018 /011. Their price: $98. Made of LEBG (low expansion borosilicate glass similar to pyrex), wavelength is 400-800nm (visible, near ir). Flatness is 1/4 wave, thickness is 10mm, parallelism is 3 arc minutes, surface quality is precision 60-40 scratch and dig. Unused!