The APS has an excellent article on gumming. Does not specifically address Flat plate gumming, but does give references for further research.
"https://stamps.org/news/c/collecting-insights/cat/exploring-stamps/post/gum-basics"
"Appears gum was added after printing."
I thought I knew, then I checked my resources to confirm it. My resources do not address at what stage gum is applied to flat press printed stamps (in my case United States Stamps 1847-1914).
In the Scott Cat. in the "Introduction" in various sections the stages of flat press printed stamps is discussed, in another section gum is discussed for flat press printed stamps, but Scott Cat. Introduction does not indicate at what stage the gum is applied.
Then I checked "The 19th Centry Postage Stamps of the United States Vol. #1" by Brookman 1947 edition, again the gum at what stage is applied to flat plate printed stamps is not discussed. (This book does go into detail with actual excellent pictures, how flat press printed stamps are printed.)
I'm asking the StampoRama members "Fractual" resources that explaines at what stage of flat press printed United Stamps stmps 1847-1914 is gum applied)?
re: Gum, When it is Applied to Flat Press Printed Stamps
The APS has an excellent article on gumming. Does not specifically address Flat plate gumming, but does give references for further research.
"https://stamps.org/news/c/collecting-insights/cat/exploring-stamps/post/gum-basics"
re: Gum, When it is Applied to Flat Press Printed Stamps
"Appears gum was added after printing."