Bringing this back to the top as I want to share some new information. I still assert, by what has been shared on this site, that the TT build date loosely follow the calendar, yet castings/stampings are exact. If you recall the OP asked if a 9/16 date axle build would work with his 9c build date car? I responded that I believe it would (I have a very close component dated car).
Fast forward. A discussion between JKZ27, via PM, and myself ensued were we discussed 12B build and VIN vs. X code start up. I cited a couple of cars I know of or found to show the VIN vs.TT vs. Date of Production.
The point I'll make is, at 912 cars per day, and if my car was on the last possible day of 12B (Friday,13th), then there would be 8-9 more production days in the year (excluding Christmas eve/day and N.Y. eve). or 9 X 912 = 8208, 8 X 912 = 7296. a week of production = 5 x 912 = 4560, How could a 12C car be built 6000 past mine (over a week)?
JKZ27's 12B Nor N558XXX Early in week, no X-code
My car is a 12B Nor N563XXX almost 5000 later than JKZ27's w/X-Code
a friend's SS is 12C Nor N569XXX About 6000 after my car
An Ebay car is 12D Nor N570XXX Less than a 1000 after my friends car and less than 500 after published E.O.M. figure (almost 7300 after my car)
CRGs published info of end of Mo. Prod. Figure for Dec. 68 Nor. = 569987 . Here, clearly the 12D car's # is after the last Dec. 68 #. The 12C is within a few hundred of end of month.
I theorize that end of Month, and year in this case, may not have counted the last week of year, Dec 25-31 ( actually 22-28). A lot of plants shut down for maintenance during this time. Start of 4th week was probably only one day (Dec.31st) and the Quantity produced was put on the next month's production total (Jan 69).