I came to the same conclusion a couple weeks ago when I was exchanging emails with Missouri and Alabama about the flag.
Just out of curiosity, how did you arrive at the conclusion?
My source was a period newspaper report that an officer of the 1st Maryland Cavalry (Potomac Legion) and his patrol found it near Chantilly, VA in the aftermath of the Maryland Campaign. Union cavalry attached to Sigel's Third Corps was very active in northern Virginia as the mryland Campaign ensued.
Greg Biggs