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Developed in the 1870s, the Hectograph (also spelled Hektograph) was an early document copying process which used a tray of gelatine to transfer ink from a master sheet onto copy pages. Extremely simple and easy to construct under primitive conditions, they were extensively used by underground political groups and prisoners of war to secretly print documents. 

SOURCES:
https://blog.lib.uiowa.edu/speccoll/2013/07/16/what-the-hectograph/
https://www.tandfonline.com/doi/pdf/10.1179/bac.1997.22.1.013#:~:text=Although%20it%20was%20started%20by,beginning%20of%20the%2020th%20century.&text=describing%20the%20printing%20process%20as,it%20as%20the%20hectograph%20process.
https://rhollick.wordpress.com/2020/01/23/hectograph/