Can Someone Teach About The 9x1-E1 Combo?

I've always been attracted to this design. But I'd like to know more about it. Does someone here know the history? The Scott catalog describes the top image as a proof. What occasioned a proof and an essay on the same sheet?

https://stampauctionnetwork.com/X/0920/1.jpg

*** Moved by Staff to a more appropriate forum. ***

See the first frame of Mark Schwartz's exhibit of the NY Postmaster Provisional hosted on the US Philatelic Classics Society webpagehttps://www.uspcs.org/resource-cent...provisional/

Thanks. This exhibit describes this as a die proof. Is there a term for a proof made after the stamp was in circulation? Post-production proof or something like that? It is confusing that the word "proof" is used for stamps made before the postage stamp as well as afterwards. e.g., the plate of 9 was made before the final stamp die while this combo was made using the "1879" state of the die. Then on top of that it is listed as an essay. Sometimes I think the terms are intentionally confusing.