Which innovative product design feature did Penguin Books introduce in 1935 that revolutionized the publishing industry and created its iconic brand identity?

In the world of publishing, some product innovations create entire cultural phenomena. Penguin Books revolutionized reading culture in the mid-20th century by making quality literature accessible to the masses through an innovative product approach. This reinvention of how books were marketed, designed, and distributed transformed an industry and democratized literature. Test your knowledge about this landmark product innovation in publishing history!

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Standardized color-coding system for different genres (orange for fiction, green for crime, blue for biography, etc.)
First publisher to implement digital watermarking to prevent unauthorized copying
Introduction of the hardcover-only premium book club subscription model
First to implement regional pricing based on local economic conditions