Not a NOR 67 but our 01B 68 NOR had this date stamping. Of the 68 cars I've reviewed the date code was present but have noted different fonts (different stamps) as I'm sure they were doing a lot of wiper motors per day and utilized several stampers.
So far most of the 67 Los cars have the stamp, but are different than your 68 as day is separated from year. Need larger sample of 67 Nor motors but so far no stamp.
Just from a production quality control standpoint I find it hard to believe that Nor motors did not have the stamp.
Until more people step forward with non restored, original Nor 67 motors, with or w/o stamps, the answer will have to be maybe.