georg,
there were no regulated/enforced guidelines concerning the materials used for manufacturing ek's during ww1 and the postwars years, except for the dimensions of the cross. it's highly likely that the scarcity of metal during a wartime economy and the following postwar recession dictated that the many makers had to implement whatever they could obtain at any specific time. as a result, one can find crosses with cores made out of iron, brass, steel, zinc, german silver, tombac, and of course one-piece ek's that have no separate core.
and on the other side of the spectrum: cores made of glass: a rare luxury item!
hope this helps a bit