With the new series “Gamme 2020,” The Banque des États de l'Afrique Centrale (BEAC) abandoned the use of a letter to indicate for which country the banknote was printed.
“D6” only indicates the sub-series of this 500 CFA note. Nothing more. It does not indicate a country of the BEAC's monetary zone.
Likewise, the prefix letter of the Euro banknotes of the second series does not indicate a country. It indicates the printer where the note was made. It does not indicate for which country, National Central Bank resp., the note was printed.