Pre 70 OE's should be in the above 7 digit format.
Some good readings concerning these era tires on the Shelby forum ,search around a bit. I have not read in depth on the subject, just enough to decipher year and month for my tires, which I already knew were original to the car.
This being somewhat rare, I will in the near future post better images of all 5.
Per P.O.P and NCRS production date on my Z was May 1, as I began documenting all my date codes I have been somewhat surprised how close to production the parts match eg; tires April, Engine 4/23 Rear end 4/24 Trans 3/21 etc...
At one time they built E250's and Thunderbirds in my home town (literally the buildings were sold for scrap price a few years ago ,sad) As a kid I would see rail cars and semi's of parts being delivered. Huge rows of stacked parts on pallets were in and outside the plant ( south side) at the end (north side) a constant flow of cars would cross over a bridge and be parked awaiting shipment near the lake. Really have to appreciate on a grand scale how production remained in sync....without cell phones and the internet. ( They also built ships ,cranes, electrical parts. Manufactured Steel and seamless pipe) Around 500 manufacturing jobs remain in a city of 68,000 now. Seamless pipe.