Jim,
I looked at Datamp, as I'm sure you did, and found nothing. Also looked at Google patents a couple ways and found nothing there.
It's possible that it was never patented. Back then it was not unknown for inventors to mark tools with a patent date that didn't exist. It would keep the competition down. Also it was much more difficult to check on whether a patent had actually been issued, not like today with all the patents on the internet.
Mike