This is a pattern from Harriet's TCS. No. 76 1700's Corset - Adjustable. Copyright 1987 by Harriet A Engler - Tailoring - Custom Sewing, pattern is used, uncut and still with factory folds, envelope is a manila envelope - it looks like that is how they sold it.
You can look up the website for Harriet's TCS which is the maker of the pattern.