guess I should chime in on this one ..........the Wgr 21 or BR 21 officially known was used in april of 43, in JG 1 and JG 11. later JG 26 used them.
JV 44 never had them at all, JG 7 had a few jets flying experimentally with them under the nose which was a joke and they tore them of and dumped them in the junk pile. the Sturm units 12.Sturm/JG 3 used a similar device for a couple of weeks with it faced rearward but it was a failure and they pulled it off, one staffel in JG 300 did the same with the same negative results.
Twin engine destroyers, well everyone in the Reich defense used them, the schools, ZG's 101, ZG's 1, 26 and 76, in fact ZG 1 used them into August of 44. Bf 110G-2's and Me 410A and B's attacked from all angles rear, up, below and for the sides.