Eric Partridge in his Dictionary of Slang and Unconventional English (1st ed.) says for square-head, or squarehead: "A Scandinavian, or especially in the Great War (i.e., World War I) and after, a German: colloquial: late 19th - 20th century. Ex (i.e., from) the shape of head. — 2. Earlier, a free immigrant: Australia: ca. 1870 - 1890. — 3. In cant (i.e., language of the underworld), an honest man: mid 19th - 20th century; obsolete." (I've expanded his abbreviations and added explanations in parentheses.) Hope this helps. I think the third definition is from square == true, as in "a square deal". I assume that AlimaeHP's "square" helmets are the ones that the German army wore from late WW1 until the end of WW2 (ca. 1917 - 1945). (They look very much like the helmets worn today by US troops, with the back half of the helmet lower on the head than the front, covering the ears.) Before that in WW1, the German army wore the Pickelhaube which is a small leather helmet with a metal "spike" (which for dress occasions was used to hold some decorative horse hair). And welcome to AWADtalk.
