You have several "build dates" to contend with. Working backwards:
- The car assembly line scheduled build date. This can be as much as a couple of weeks from the actual build date and cannot be verified on 64 thru 66 Mustangs except with a rare build sheet. That scheduled date is on the door data tag, or can be approximated by the VIN comparing the VIN to other car's scheduled dates.
- The engine assembly date. This date is stamped on a "boss" on the engine block near the distributor. See picture. It is in the format of year (number), month (letter), day of month (number) and an inspector or assembly worker code - 5H26Z. It will be a week, or longer, before the scheduled build date.
- Component build dates. Every item that goes on an engine should have a date code prior to the engine assembly date. A casting date (block, manifolds, etc), stamped date (distributor) or tag (carburetor) are examples. A week to 4 or 5 weeks would be correct.
I suppose that in unusual circumstances a carburetor or distributor that failed at final test could be replaced with a later dated item but that would be difficult to prove.
Jim