If I recall correctly, the current date coding system went into affect in 1968 or so. At that time it was a 3 digit week/week/year layout, but that date code was part of a 10 digit TIN (Tire Identification Number), with the date code at the end. The system was modified in the mid 1900's to add a decade arrow, then further modified, starting in mid 1999 and completed in 2000, to a 4 digit system (week/week/year/year), which how it appears now.
Prior to that, each manufacturer had their own system (or not!) - and there was no commonality between manufacturers.
So a BFG tire from the mid sixties would have whatever BFG was using back then.
I'll bet that no one knows what that system was - given that Goodrich merged with Uniroyal in 1987, and was later acquired by Michelin in 1990, so whatever was used back then was probably lost in the shuffle.
Just an FYI, the current TIN system is a 11 to 13 digit system with the first 2 or 3 digits indicating the plant of manufacture (starting in April, 2015), the second group of 2 digits is a size code (manufacturers must use a system of some sort and many use the USTMA coding), the next 3 or 4 digits are the type coding (only the manufacturer knows what those mean - if anything!), and the last 4 digits are the date code. In April of 2025, the 3 digit plant code becomes mandatory.
See my web page for more details
Barry's Tire Tech - DOT Codes