Those can't be printing plates in the sense that they were used to make the final image on paper/cardboard, because the image would be backward.
I know next to nothing about printing but maybe they were used in some sort of intermediate stage in the printing process.
The only analogy I can think of is the coining process. A master hub is made (reverse image), which creates working hubs (actual image), which are used to create working dies (reverse image), which create coins (actual image).
Maybe someone with printing knowledge can explain what they are.
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