Hello Bill
It is my understanding that the R6 designation meant that the aircraft had the provision to be fitted with a pair of 21-cm W.Gr. 21 rocket tubes under the wings which were supposed to be used to break up the American four-engined bomber formations so that the defending fighters could attack separated and thus vulnerable bombers. These rockets were also used in the ground-attack role on the Invasion Front. The image below is of a Fw 190 A-8/R6 of I./JG 26 deployed in such a fashion. In all likelihood the wing guns were removed from the Mistel components simply to decrease weight. I hope this helps you.
Horrido!
Leo