This one is a model of a very rare SPG. During desert warfare PIII's got blown up so they cut out the upper hull, and welded a floor and upper armour on, a SIG33 artillery gun was then welded inside. It was the forerunner to the Stugs.
It's just an all plastic RX18 powered runabout, it's very light and goes anywhere without complaint. I might re-do the gun one day, and as the Clark boards are the same size as an RX18 there would now be room to make it fully functional.
I have to admit I initially thought you'd based this on the Bison II, which was actually a six-wheeled, Pz Kpfw II based, self propelled gun using the SIG33, but I've just found the original picture of the Afrika Korps Pz Kpfw III conversion in the "Rommel's Funnies" book
Looks spot on
I've been trying to source the StuIG 33B conversion, but no-one seems to supply it any more