Thanks for the additional info - helps:)
Not sure the purpose of these marks that were added at Dearborn that are over the black - not to be confused with the ones under that were likely pallet data
What your seeing (rest is likely covered by the sound deadener) is the bottom row of what should be three rows of two numbers each. So far the numbers used in the pattern were 0-6 normally applied in white or yellow. Yours are a bit thicker (worker must not have sharpened the crayon for that car) than other examples. Marks can be found on the drivers side but also (a few in the sampling were on the drivers side - likely a different shift or different worker that shift) Have only seen these in 65 Dearborn production
That's all I have on the subject currently