Ferdy Mayne has dialogue as Karl, Herzog von Grauberg during the opening credits of every episode and is often featured in the opening scene (which were probably all filmed on the same day). However, he is only listed as part of the cast sporadically.
Piet van Friesen, who has no other credits on the IMDb, is listed as co-writer on the 13 episodes based on the original Dutch screenplays by Gerard Soeteman (who does not receive a credit at all). This would suggest that Van Friesen was either the translator or a pseudonym used for Soeteman.