My latest US m1, nothing special here, but I just wanted to ask about the number stamped inside. I am almost certain this dates from the 1960's or perhaps 1970's, but I have not come across this marking '31' on any others and can not appear to find reference to it. The helmet appears to be a standard Vietnam era US m1, the buckles are marked with an anchor. Any help on the '31' mark inside rim will be much appreciated. Thanks.
(Sorry the pic of buckle is a bit blurred)