Tintin had been available in the form of waterslide transfer sets since 1954, & you can still find these, in their distinctive red-&-white livery, on eBay. But it wasn't until 1978 that Willeb produced this range of sets in dry rub-down transfer form.
Although there is no mention of Letraset, given the date & the label "Printed in Italy" (in English), these are obviously Letraset transfers printed at Sodecor. Several Tintin books were available, with the serial numbers marked on the front cover as well as the transfer sheet, & taking the form 1926 & followed by a letter. The three shown here are 1926 A, 1926 B & 1926 D.
For more Hergé, see the classic 1968 Tintin Promotions page; & for other Belgian / French Bande Dessinée characters, visit the Dargaud Décotransfert page.
Le Secret De La Licorne (Secret of the Unicorn)
I like the fact that the cover artwork is taken from someone having genuinely rubbed the transfers onto the background, as you can tell by (e.g.) being able to see through Captain Haddock's hat.
Les Cigares Du Pharaon (Cigars of the Pharoah)
Le Lotus Bleu (The Blue Lotus)
Back cover for all titles:
Tintin Promotions ↑
Dargaud Décotransfert ↑
La Vache Qui Rit: "A La Conquête de l'Espace" ↑