Would guess that it might be a general standard though might differ from manufacture to manufacture.
Only research and data will help us figure this out - I expect
I did a date analysis on three years, 65, 66 and 67, for working days in a possible fifth week.
1965 had four months with fifth work week days: March - 3, May - 1, August - 2, November - 2.
1966 had four months with fifth work week days: January - 1, May - 2, August - 3, October - 1.
1967 had three months with fifth work week days: May - 3, July - 1, October - 2.
I did not consider holidays, weekends or plant closures. The number of days is maximum, no more than 3 in any fifth week is possible no matter what day of the work week is used as a starting point. There will be a maximum of 4 or minimum or 3 fifth work weeks in any given year.
Jim