Looking at a 1968 (which was a leap year) calendar, I wonder if the 53 stamp represents the short and final week in December, where Monday was the 30th and Tuesday the 31st?
I suspect Pete is right; I had that thought after I posted my prior post questioning what the 53 meant... 7x52 is only 364 days, so there's always a day or two 'extra' above 52 weeks, and with the first weeks of a calendar year sometimes being shorter, that can make for a year having 53 'manufacturing weeks'...
