Keith,
I hope this helps you.

"The yellow flag with the green H was adopted as a hospital flag by the U.S. Army under a General Order issued on 4 January 1864. Prior to that, as far as I can determine, the only flag used was a plain yellow flag. I don't believe that the CS Army ever used anything other than the unmarked yellow flag as a hospital flag.
Randy Young and Devereaux Cannon, 10 October 1998

The regulations for the Medical Guidon specified that was to be 14 inches wide and 28 inches long, i.e., proportioned 1:2. The color illustrations in my book, Flags of the Union, correctly illustrates the guidon having its one inch green border on only the 3 outside edges. However, in the black and white line drawing the illustration erroneously shows the border on the hoist edge as well.
Devereaux D. Cannon, Jr., 2 April 2001"
FC