5A and 6A are the most common 65 and 66 date codes . . .
There are two stories about date codes 5A and 6A:
1 - Ford ordered a large quantity of air cleaners on those dates, but over estimated the usage and did not place another order for a long time;
2 - The fabrication drawings indicated markings there and the example was 5A and 6A so the numbers were stamped that way, sort of defeating the date code philosophy.
Jim