The US military (and NATO affiliates) do not mark ammo cases with the caliber. letters indicate WHERE it was made (like the Lake Cit Ammo Plant) and the year it was amde (1963) The odds are that is 7.62 NATO (little late for 30-06, little early for 5.56. COULD be .50 BMG. Post a photo with ammo lying besides a ruler. If there is a circle with a cross in it, means NATO standard load.
And where ARE my manners? Just noticed this is your first post- welcome! When you get a minute, swing by the intro thread and say hello.