PAT303 is right in that that E-4 got as factory-fitted armour plates both protecting the fuel tank and pilot's head. That is from Fernández-Sommerau's Messerschmitt Bf 109 Recognition Manual. The book got much Flak at 12O'Clock site when published but IMHO is a reasonable accurate book. Fernández-Sommerau also notes that armour was also extensively retrofitted to the E-1s and E-3s as a field mod. According to Kurfûrst (at 12 OCH) '...there are some pictures of armored glass fitted, the latter, however, appears to be rare; I suppose it was useful when fighting bombers, and RAF bombers seen in the daylight after France were rare, therefore, the armored glass was probably not preferred...'