Which innovative retail feature was included in the original pressing of The Beatles' 'White Album' that became a collector's item and marketing phenomenon?

The Beatles' ninth studio album, commonly known as 'The White Album' due to its plain white sleeve, contains hidden stories about its revolutionary packaging design. While album art was becoming increasingly elaborate in the late 1960s, this minimalist approach created one of the most recognizable album covers in music history. Test your knowledge about this groundbreaking design choice that influenced countless artists and established new possibilities for music packaging and branding.

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Each album had a unique serial number stamped on the cover
The cover changed color when exposed to different temperatures
It included a built-in musical chip that played a hidden melody
The vinyl itself was white to match the album cover