I just received a WW1 helmet but what baffles me is the heat stamp indicating "CU308" under the brim. I find no listing for this anywhere but I read in an older blog that "X, Y and of course "Z was used for american helmets. Is it possible the letter "U" was used before the other letters? If so, can that be a rare one issued? There is no liner or pad in the lid nor does it use the split rivets. I'm trying to upload photos but am having difficulty, but I can e-mail them.