6661 was my first set, and for months (though it seemed like years at the time) my only set. I built so many things out of those 78 pieces. https://twitter.com/mwichary/status/956261286798548992 …pic.twitter.com/kOf1p5Ttuq
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It was printed on. Stickers appear to be a later development (I only encountered them once my son started to play with LEGO).
Only ever used in this set: https://rebrickable.com/parts/4215ap09/panel-1-x-4-x-3-solid-studs-with-tv-logo-print-6661/ … There is this variant that may have used a sticker though: http://bricksetforum.com/discussion/15873/the-mysterious-german-6661-mobile-tv-studio-set …
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