FN used to use a 2 digit code for date of manufacture:
Z=1, Y=2, X=3, W=4, V=5, T=6, R=7, P=8, N=9, M=0
So my FNP has a 61B the MR (the date code). My FNX 45 Tac does not follow this system, so I don't know how to determine it from the serial on current guns. I just look at the fire case that came with the gun.