What it is Manufactured by Leitz (now Leica Microsystems) in Wetzlar, Germany, the plate is a lithographically defined reticle on low-expansion fused quartz. Seven precision apertures—numbered 1 to 6 plus a large 10 mm master—span roughly ~1-4µm(you can't see it with the naked eye) up to ~0.6 mm, giving a single artifact that serves both high-NA oil objectives and low-power video macros.
eBay