If you are talking about the marking on the elevation turret on the M-24 sniper rifle, the M118 designation refers to ammo that was formulated for the M24. The top of the turret is graduated from 100 to 1000 yards based on this ammo.
Back in the good ol' days, the Leupold Custom Shop would change your reticle for a reasonable fee. Possibly a shooter requested change after its' initial sale.