The Forster Flag was a British flag that was altered to become an American Patriot flag. It is believed to have been a British Red Ensign that was somehow captured from the British troops during their attack on Concord on 19 May 1775 and came into the possession of Samuel Forster.
The British Union that occupied the canton was removed, and in its place were sewn six short white bars on the reverse side / seven short white bars on the obverse.
Dr. Whitney Smith, Director of the Flag Research Center, in regard to the significance of the Forster Flag, made the statement that: "It is the oldest known American flag, i.e. a flag intentionally designed and used to symbolize the country." That is a very audacious statement to make in view of the fact that there is no evidence ~ other than the family's own 'history' of the flag ~ to suggest that it was intentionally designed and used to symbolize the country.
Not all historians believe that this flag was a 'captured' British flag. No flags were reported as missing from the British records for the first battle of the American Revolutionary War.
The suggestion has also been made that this flag might have been the colors of the Manchester Company of the First Essex County Militia of the Massachusetts-Bay Colony. It might never have held a British Union in the canton.