Reichststurmfahne (military banner used by the Holy Roman Emperor during ca. 1200-1350).
The Reichssturmfahne was not a flag, but a banner. Meyer's Lexikon of 1897 says s. v. Banner: " [...]    When investing a fief, the Emperor had together with this Imperial banner in black and yellow a second banner in red for the investion with penal judicature. This is probably the reason of combining the colours black, red and yellow to form a tricolor against the rules of heraldry...The Reichssturmfahne was smaller than the Imperial banner, more like a chaffron... [...] The banner of the Holy Empire bore a cross even before 1200; it is the St. George's banner... Emperor Friedrich III took the cross officially as part of the Imperial arms, but some of his successors did not use it." A. Birken, 22 Aug 2002
FOTW
Sir Iain