I have ran into the question of date before where some were dated with the normal 10,000 day calendar. But have noticed that 67 and back the date doesn't seem to be the same. I have a 67 440 HP trans and I believe that it stamped H for the plant, then the part number, followed by the date. but the date is not correct for the year. I have seen them stamped PK, H, and J before the part number. Would like to know more on the dating as well. I can try to get pictures maybe later, if that helps.
Matt