they do not use the letter I (I think cuz it can be mistaken for a 1) . so I guess your heads are day, month and they left off the year. 10J is the 10th of september, and 8K is the 8th of october. and 1972 340 or 1971 or 1972 360. I worked in a steel foundry for 10 years, and bought my first 340 car in 1977 for $500.