(with my text decoding)
Thanks to Mike Ransom for this info on Plants.
M = manufacturing plant Motor Wheel, 1 was in USA and 5 was in CANADA.
K (or KH) = manufacturing plant Kelsey-Hayes.
T (or NI) = manufacturing plant Norris Industries, maybe called Norris Technologies.
This 1968 JC wheel had the date information on the inside of the rim
The JJ is the code for the flat part that connects both sides of rim
The M1 is the plant code where the wheel was made.
The outside of the same JC wheel, with the JC code
A 1971 date coded KR wheel. The wheel was made July 21, 1971
A 1970 dated KR wheel. The wheel was made on Nov 9, 1970
(for the 1971 model year)
1973 Dated HM wheel. Date made - Dec 6, 1973
The Plant code is M5.
1976 Date coded HF wheel. Date made - June 9, 1976
Plant code - M5
1977 Date Coded KM wheel. Date made - Nov 16, 1977
The Month code is hidden by valve stem, it's above the valve stem hole
Plant Code - M5