I have never heard of Z in Royal First fare, L R in Royal Silk, N in Economy according to TG's booking office in Thailand,

First: F A P
Gold Upgrade First: P
Mileage Upgrade and Redemption First: O

Business: C D J
Gold Upgrade Business: J
Mileage Upgrade and Redemption Business: I

Premium Economy: U
Gold Upgrade Premium Economy: U
Mileage Upgrade and Redemption Premium Economy: L

Economy: Y B M H Q (S T K)
Economy Group (50% mileage): G
Economy Discounted (25% mileage): V W
Mileage Redemption Economy: X

N.B. Premium Economy is sold on CPH and ARN only. Economy S T K fare is not used by TG booking office but appear on TG's 2012 handbook.

Please check and let me know whether this is correct or not.