This is what the Dead nuts website has to say
Backup lights changed to a thicker lens starting on 2-28-69 (per Ed Meyer) by adding a second thin 69 lens. The Master parts catalog put the change over date to the thicker lens on 4-7-69. It may be impossible to say exactly when the cut over date occurred since it would hinge on when the remaining stock of thin lens was exhausted. According to Ed and the MPC you would be safe to use the thin lens before 2-28-69, double-upped lens from 2-28-69 thru 4-7-69 and the thicker lens after 4-7-69. You can find plenty of NOS 70 lenses around but it's difficult to find a set of NOS thin 69 lenses because they only serviced them for less than a year. One way to get a NOS looking set of thin 69 lenses is to find an original set of backup lights that were doubled up and use the lower lens.
My Dearborn car has a scheduled build date of April 7, The Invoice was prepared on Apr 10 and the invoice is stamped shipped on April 14. I've go one thick and One tripled up backup light.
