Its a Bayonet frog.The inside of the frog (facing rear of ammo pouch) is reinforced to stop the wire cutter on the Bayonet sheath from wearing through the ammo pouch .When the bayonet and sheath are slid into the frog the cord goes around the bayonet hilt and you tighten the toggle.The string on the sheath is supposed(Fat chance) to be fed through the eyelet on the bottom of the frog and tied off on the lower D ring to stop the sheath from coming out when you fix bayonets.When not in use IE no bayonet tuck the toggle away into the frog to stop it getting snagged on things.
PS: As a nice to know the lower size adjusting straps on each side of the BV have fastex buckles while the others don't.The y are for use when the Bergen or patrol pack waistbelts are used.You open the clips and pull up the sides of the BV to fit down over the waistbelt so the belt goes around your waist and not around the ammo pouches.