Calibrating standard pattern

• Description: Overstock from my lab. Special industrial calibrating standard. It is a heavy duty metal frame (76x76mm) that has a 42 microinch rectangle finely itched in a front glass to calibrate microscope or machinery. For more info, see backwards side in pictures.
• Condition: Please, see pictures for condition. I took pictures with around 16x magnification of the centered itched rectangle. Probably this thing saw better days, but it might be useful to you.