Also I just noticed that on the '72 chart that Doc showed us, there is a JY9/2510. So the theory of the first letter being a first year of change is disproved.
I do not see any good explanation either, wonder if they might have added the color mid year '72 or something and used the '73 coding which was probably on the books someplace.
Something similar happened with KT9 Dark Chestnut Metallic: the K is generally used for a several colors introduced at the start of the 1974 model year, but the color AND the code were already used on the 1973 Fury Special, introduced about half-way in the 1973 model year.