With Samsung models, the letters in 5th and 6th place indicate the year of manufacture.
Example model number: UE55RU7170
TV sets with the following codes have come onto the market since 2017: Q, RU, LS, R, NU, N, MU, M.
With LG models, the character in 4th place indicates the year of manufacture.
Example model number (LED): 65SK9500PUA
K = 2018, M = 2019, N = 2020, P = 2021
Example model number (OLED): 65C8PUA
8 = 2018, 9 = 2019, X = 2020, 1 = 2021