The Charts are wrong (or at least misleading) and have to be taken with a grain of salt, but those are the numbers reported by GM as the final VIN that was assembled each month. Look at May of 68, they reportedly made 39000 cars in May. Norwood built 912 Cars a day, every day, so with a 21 day work month the most they could make any single month was 19152 cars if everything went perfectly. So did the plant lie in May to get some kind of performance bonus, or was it a typo that they had to reconcile by fudging the rest of the year numbers, so they didn't get penalized for lack of performance, who knows.