Wasn't the
Rüstsatz 6 (R6) modification the Werfergranate?
Sometimes refered to as BR 21, a version of the 21cm Nebelwerfer rocket mounted on aircraft (notably FW-190) to attack USAF Bomber "boxes".
http://www.luftarchiv.de/index.htm?/flugkorper/wgr21.htm (in german). Or am I way off base here?