Which innovative packaging design feature was first introduced by Merriam-Webster in the 1970s that revolutionized dictionary sales and became an industry standard?

In the world of language reference materials, packaging design has evolved dramatically over the decades. Dictionary publishers have had to adapt their packaging strategies to remain relevant in an increasingly digital world. This trivia question focuses on a pivotal moment in dictionary packaging history that revolutionized how these reference tools were marketed and distributed to consumers.

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The 'thumb index' - indentations along the edge allowing quick access to letter sections
The shrink-wrapped paperback dictionary with visible sample pages
The 'dictionary + thesaurus' combo package with coordinated cover designs
The clear plastic carrying case with built-in magnifying bookmark