What is a "Corrective Slide"?

Dear folks,

A bookmark-size piece of clear plastic with words in Gregg shorthand fell out of a second-hand book of shorthand exercises I was looking at on my shelves.  The caption on the top says "Gregg Shorthand Corrective Slide" but is puzzling to me.  Has anyone ever seen one of these gizmos before, and if so, do you know what purpose they served?  The shorthand is written in red ink against the clear plastic.  There are four words in each row and there are fourteen rows from top to bottom.  Is it a type of "addenda" or way to rectify mistakes on the plates?  Baffling to say the least, but perhaps someone out there may have a better idea than I as to what this can be.  Thanks!  Jim

(by jim for everyone)