That "B" sticker is one of the final inspection OK stickers stuck on the inside of the windshield at the end of the Final Line and after roll-test to aid in routing the car through Final Process and to the shipping line. A car ready to ship would have had four of them - "M" for Mechanical, "P" for Paint, "B" for Body, and "RT" for Roll-Test. Those stickers were removed on the shipping line - it probably fell on the floor and was still in the car when it was delivered.