I am betting it's a tatum, where you put the original specification sheet underneath and then while doing work o whatever are doing, you use grease pencil to make notes, mark checklist etc., then at end of day you make photo copy of the thing with again original spec sheet and modifications. Tomorrow you use the photocopy, original stays in the file and you have cleaned the grease pencil markings off the glass. Kind of pre-scanner, not everyone owned a computer days.
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