[JohnZ]

The DOT required a TIN (Tire Identification Number) on all tires beginning in 1968, but at that time, the manufacturers were permitted to use their own proprietary coding formats. The regulation was modified in late 1970, and DOT mandated a standard format for the coding beginning in May, 1971, with the last three digits signifying the week of the year and the year. The format was revised again in 1999 so the last <span style="font-weight: bold">four</span> digits signified the date, with the last two being the year.

As far as I know, for the proprietary codings used from 1968-early 1971, only the Goodyear codes are known (thanks to Kelsey Tire).