In Vol. 1 of "Advance the Colors!" on page 64 there is a brief description and photograph of a designating flag. A reference cited for this flag is H. M. Madaus' article "McClellen's System of Designating Flags, Spring-Fall 1862", Military Collector & Historian 17 (Spring 1965), pages 1-14.

Does anyone have this article and would be willing to scan it or photocopy it for me?

There is another example of of a designating flag at the New York State Military Museum and it shown below. Also included is a link to more information about this flag.

Ed