I'm considering an M1 helmet that has this unit mark on the inside of the shell. The picture looks brown, but the helmet is the dark green you'd expect. I think the seller may have altered the hue to make the stamp more visible.
I'm just not sure if this unit stamp looks WW2 or not, or could have been Korean war as the 31st served in both in the Pacific. The 31st infantry saw just 4 months of action as the Japanese invaded the Philippines in WW2. They were fighting at Bataan, where they surrendered and participated in the infamous Bataan Death March.