Here is a quote from JohnZ's post on yenkos.net:
Yup, that's a final inspection ticket, and the stickers that went on the inside of the windshield when each category was bought-off at the end of the line or after roll-test. There were usually four stickers - "M" for Mechanical, "P" for Paint, "B" for Body, and "RT" for Roll-Test. It was the job of one of the inspectors on the Shipping Line to remove the stickers and the inspection ticket before the car was handed-off to the carrier at the end of the line. Each sticker normally had an inspector's ID punch through it to identify the inspector who gave that category its final OK.
