Thanks Jeff - Any idea what the 5426 numbers represent?
Apparently so method or practice to identify the panel or body at some point in the line
Both the 5426 and the "7" on the back of the battery tray would have been written on the bare sheet metal, before the black paint was applied. I tried to take a photo of the "7", but it didn't turn out that great, so I've posted an enhanced photo below as well...
Thanks for the additional information - helps allot
Since the 7 is on bare metal then I believe its an identification written on the panel by the supplier (like the top panel on a secured pallet or similar panels) that identified the stack for those workers or handling personnel. The marks seem to show up on about one in fifty or one hundred cars and are typically found on the front drivers and or passenger panels. Have not figured out a pattern to the coding but I do have 7's used in some of them from 67, 68 and 69 production. Typically there are two lines of letters and numbers in the pattern. One of these patterns is on the 64-65 Dearborn coupe shown in the bunch of assembly line pictures that Mustang Monthly published many years ago (believe many are on site here). Its the picture with the guy guiding the body - the one (if you've ever met him:) looks like Charles Turner.
Not a big surprised to find one marked this way at another plant since (as mentioned above) I believe its a stamping plant thing for the individual panel and those plants supplied many of the plants.
Just my conclusion at this point