Small Scale Military Headquarters

I've got a 1:18 Sd. Kfz 251/22 Hanomag / PaK 40 and in the accessories bag there is a storage locker with shells that looks like it has a seat back attached to it.

I can't figure out where this goes?, does in fit in the Hanomag or just suppose to be displayed on the outside?

i believe it just sits on one of the seats....the idea being that soldiers would sit on it, rather than the actual seat, while the gun and ammo were being transported. I guess it was to add a bit of comfort over long distance hauls. I find it very odd, myself..

If it goes in the Hanomag I sure can't figure where.