Thank you for your reply and insight!Letter is Year. Some letters are not used. This replaces the old 60s and 70s method where year was a digit, as in 5R09K165xxx where 5 is 1965, R is San Jose, 09 Is fastback, K is HiPo, etc. The new method was mandated when they began to use country & manufacturers codes, example: 1FATP8JZ0G5524756, where 1 is USA, F is Ford, T is R model (6 is street) G is 2016, 552 is GT 350 platform, etc. I'm sure you can Google this stuff if you want to get deeper.