Tim has the bullseye !!! (and a lot faster than I had expected).
View attachment 71649
The missing part is the forward flange (and all of the "cartridge bits" naturally

).
So far I've mostly encountered The Pz b version in pics and real life, and few enough of them!! This would have been a live round.
Dave, of course you are the member whose avatar I was referring to. Iff I'm not mistaken your avatar refers to the "counterpart" of the SG113 designed to combat bombers, a single shot, recoilless weapon firing an aluminum/light alloy HE/Minen round, correct ?
Over to you Tim
Regards,
Menno.