[Mario_Fr]

Yes, the Werfergranate 21 was mounted on some planes (Fw 190, Bf 109, Bf 110 e.g.), in the OOB it`s weapon 175.

But I have no idea what Werfergranate 8r (weapon 173) is supposed to be. Also it`s only mounted on unit #130 Hs 129B-2.

In any case, the Werfergranate was not intended to be a air-ground rocket. As you state correctly, it should break up USAF bomber boxes.