Hello Waroff,
Now, it sounds clear as the two instruments show respectively the 'anémomètre d'atterrissage' and the 'Contrôleur ACDFK anémomètre' (up to 500km/h, as the MS406 doesn't reach that speed, except in diving!) which is a part of the 'Contrôleur de Vol' so I was wrong about the 'blackening theory'
, which was, in fact, the back wall of the instrument. Unfortunately I cannot go further in the cleaning as the decals are so fragile.
Best regards
ClinA-78