Uwe is spot on. AC Cars chassis tags, whether made for AC or after-market suppliers, tell us nothing of the origin of the car unless the car itself has 'continuous history' as a car since original manufacture.
If it has continuous history since original manufacture by Factory Five, it's a Factory Five. If it has continuous history since original manufacture by AC (either under the Shelby contract or for sale through AC's agents), it's an AC.
RS