All the german Sturmgeshutz variants hade sheet steel plating along it's sides.... Only on later variants side skirts where mandatory. But that doesn´t mean that earlier models were not upgraded.
did the same thing... just a lot more weight. Originally the side skirt´s were not added to stop hollow charge´s. They where meant to stop large caliber anti armor gun´s like russian 14,5mm PTRS or the british .55 Boys. That they also helped against hollow charge was a positive side effect (The theory behind hollow charges was then more or less unknown.)which proved useful in the later year´s of the war.
Also, thinking about it, the slat system probably dissipates a lot of the impact's shockwave