The Electorate of Brandenburg used since Middle Age a coat of arms consisting of a red eagle on white background, with yellow beak, talons and clover stalks. Naval flags showing that same device are reported from as early as XVII century, although slight differences can be observed from one report to another (either crowned or uncrowned eagle, escutcheon on its chest present or absent, etc.)
When in 1701 Brandenburg became the Kingdom of Prussia, red eagle flags became obsolescent and were replaced by prussian black eagles.
Source: «Flags of The World» (FOTW) website:Electorate of Brandenburg 1356-1701 (Germany)
Sizes: ensign W12 x L18 mm, mast flags W8 x L12 mm, jack W6 x L9 mm, pennant W4 x L48 mm.