Thank you for your prompt reply! That was the puzzle solution!
I did not know the English expression for "postal stationery"!
You say "embossed envelope"?
I think since this stamp has been cut out, does it no longer have any value?
I still have such a cut.
I now found examples and exact numbering on this website:http://dubinweb.com/USstampedenvelopes/0.02.html
My cut is the design U 93 and U 86
In german we say "Ganzsache " = "postal stationery" - see Picture
This is my Website with US-Stamp "one Cent" "Washington / Franklin" issue:***Please no 'drive-by' links***
(use Translation buttom!)