Its a type of model from Trijicon ACOG.
This model of ACOG was originally intended for use with the M240 medium machine gun. The open stadia in the reticle are for acquiring targets at ranges calibrated for the use of standard M80 Ball 7.62x51mm ammunition - the most common ammunition used by the U.S. military with the M240 series machine guns. This optic will still work extremely well with .308 Win / 7.62x51mm rifles with 16-18" barrels (AR-10, SCAR-17S, PTR 91, Sig 716 Patrol, etc.) and MILSPEC M80 ammunition, but do not expect competition-level precision.
M240 is a designation for 308 7.62! You can see M240 designation as you peer into the eye piece looking down!