The American Journal of philately, in January, 1869, in an article by Dr. Petrie, gave the first correct account of it.
"History of the Postage Stamps of the United States of America"
John Kerr Tiffany
From a letter of Mr. Mitchell's, printed in the American Journal of philately in May, 1871, it further appears that Mr. Mitchell permitted parties to bring their own envelopes to be stamped.
"History of the Postage Stamps of the United States of America"
John Kerr Tiffany