I have a son who studies Greek, so my first thought was Sigma. The cross is also in a common Eastern Christian shape. It’s just a thought ... but troops were moved around a lot in WW2, and your father might have picked up an artifact from the eastern Mediterranean theater in Western Europe, if a unit had been in both.
As to the possibility of its being a WWI piece, perhaps he found it in a shop in France, or crossed paths with a French veteran of the Great War, or another American who was a collector. I was surprised, in some recent reading about the Normandy invasion, at how much contact individual American troops had with French civilians.
Or, perhaps, he got it from a German veteran of WWI who later fought in WWII, either alive,or not. No way of really knowing...
the infowarrior