ok so i'm not that up to date on any of this but i wonder. you said you need written proof, some forn of documentation. well, you are right here on ancestry.com, which HAS written documentation. I started by searching for my great grandfather. eventually i just kept on going down the line with the fathers. i came to a point where i saw our dear friend Matthais. it's all right here...

Name: Matthias BRANDENBURG
Sex: M
Birth: Abt 1740 in Berlin, Prussia
Death: 20 Nov 1806-1807 in Clark Co, Kentucky
Note:
! Brandenburg Book
! Birth-Marriage-Death:Pers. comm.;Otto Harrington;Allen,OK;Jan 1991
! CENSUS: 1790; Hampshire Co.,VA;1782 & 1784; List of Abreham Johnson; 12
whites; One Dwelling
! Frontiersman and farmer. Immigrated to the British Colonies in North America, sailing from Amsterdam aboard the vessel TWO BROTHERS, landing at the port of Philadelphia, Pennsylvania Colony. 15-9-1752. Established himself with the German Community of Frederick County, Maryland, there remaining until 1773. Resided within Hampshire County, Virginia, 1773-1787. Recognized for assisting the American Colonists during their war for Independence, having donated a blanket to a patriot soldier. Sold estates within Hampshire County and removed further east, to the Kentucky Territory of Virginia. Settled southeastern Mason County, Kentucky territory, which later became Clark County, commonwealth of Kentucky. Inhabited Clark County until his death. Murdered by William Hardin.
! WILL is in Clark Co. Courthouse, pp. 240-259.
!WILL: Frederick County, Maryland, (A1)528-529.
!SOURCE: Brandenburg, E Craig:THE BRANDENBURG FAMILY [of] HARRISON CO, INDIANA, 1752-1980.
!SOURCE: FHC #6100121.
!SOURCE: Deatrich, B A:THE BRANDENBURG FAMILY, (Louisville, Kentucky, c.1950).
NATI: 1/2 Brother To Fredrick The 2ND Of Prussia

Father: Wilhelm Heinrich BRANDENBURG b: Abt 1722

Fredrick William, 1st king of Prussia.