The original drawings for these covers are made by Hergé in black ink with high quality nibs bought in London. Some areas are filled with a light texture in blue pencil and the image is often embellished with white gouache.
The drawing for the cover of July 20, 1939, 22 x 25 cm, was sold for € 540K including premium by Millon on December 14, 2014. The theme is royal and emotional: the devoted Milou (Snowy) brings back to the king the scepter stolen by the bandits.
On June 28 in Brussels, Banque Dessinée sells the cover art of December 1, 1938, 22 x 22 cm, lot 484 estimated € 400K, titled in pencil Un Faux Pas (a false move). Here is the link to the website of the auction house.
We are still at the beginning of King Ottokar's Sceptre whose publication had begun on 4 August 1938. Tintin stumbles voluntarily from the removable bridge of an aircraft in order to grab the beard of his traveling companion. Unlike his hypothesis, it is not a fake beard and the old scientist bent under the shock is expressing an intense pain. Within the door of the plane, Milou does not know whether he has to intervene.
Tintin is always right in Hergé's stories. The following of that story indeed confirmed that the old character is a conspirator, despite his real beard.