To me a steinkern is the internal sediment filling of a fossil like a gastropod or bivalve. It is a cast. A mold as far as I am concerned is an outer concave thing. For example when you are making a fossil replica you pour a latex or other flexible substance around the real fossil. When the latex is cured, you cut through it and remove the real fossil, and then you pour plastic (or similar substance) into the void. When the void filling is hardened you remove the fossil replica (the cast) from the surrounding mold.
There may be other interpretations, but this is mine!