I just recently aquired a WW1 american helmet with the heat stamping under the brim indicating "UC 308". Any chance that this was an american manufacturer? I know 'Z' is the most common because I have a few of those, but I read somewhere that the letters "X" and "Y" was also used. I find no info at all on "U". Was briefly listed on another blog that it was american also. If so, is it rare? I know the "X" is quite rare. I have seen "Y" on occasions. As for "ZA, ZC, ZD and ZJ', those are quite dominate. So can the letter "U" be more rare than the "X"? I'll include pics. Thanks.