If you see other chassis plates with letters, they are as follows -
VP = verification prototype
TT = tooling trial
PP = pilot production
K = 2019 model year
L = 2020 model year
Owners can have cars with PP and MP plates, in addition to the K and L. PP and MP cars are saleable units at the production configuration, but used before the main production run as media and evaluation cars.
More information about each Bulllitt's serialization strategy can be found in Mustang Bullitt - Generations