Optical flat, with case, glass / leather / velvet / metal, possibly made by Lensmaster, Illinois, United States of America, c. 1960, used by Ross Simpson Engineering, Sydney, New South Wales, Australia, 1963-2007
The optical flat is a thick clear glass disc with polished flat top and bottom and frosted cylindrical side. Three black arrows, all pointing the same direction, are lightly etched into the side of the disc. The flat is stored in a square grey leather covered case with black velvet lining. The flat fits into a cylindrical recess in the case. The case has two hinges at the back and two latches at the front which are opened by horizontally sliding outward spring loaded buttons. Marks on lid.
The cover of the case is imprinted with the manufacturer's name or brand but this is not clearly legible. It might read 'Lensmaster'.
Underneath the name/brand is an illegible word which might be the company name followed by "Company". Next line is the city (illegible - looks like "Gilove"), "Illinois".
There is a silver sticker on the upper left of the top outside of the case from the "National Measurement Laboratory" with the CSIRO logo and "CSIRO" under the logo. "Date of Calibration" is printed and "June 1995" is handwritten. "Report No:" is printed and "RS25575" is handwritten.
Above the CSIRO sticker is a black Dymo label imprinted "M21407".