Jeff, although I've never compared the "date stamp" to the actual build date of a vehicle, this is what I believe it to be. Your statement that it may be a stamping date, or date code, would be correct but for the fact that fox body cars used date codes.
I have found codes stamped into the sheetmetal on fenders, doors, hoods and hatches. As for paint checks, when we pulled the carpet on my 1E/9L t-top we found what appeared to be an assembly line check sheet for paint. It was pretty deteriorated but that's what it looked like.
I've done photo studys of some very low mileage cars and never seen anything like the "Henry" stamps you see on 65-69 cars.