I stick by my previously submitted quote from Grabau. Your clam on the right in your image is an internal mold, i.e., it was made from the original shell by an infilling of mud. This is a steinkern. If you coated it with a silicone compound you will have a cast of the original shell interior.
Let’s say the original shell was buried but did not get infilled at all. Further the aragonitic shell dissolves through time but the hollow cavity remains. That hollow cavity has the impression of the outside ornamentation of the shell and would be an external mold. If you pour silicone compound into that cavity you will reproduce the original shell morphology, i.e., a cast of the original shell’s exterior. If this cavity were filled naturally, the result would be a natural cast of the shell. Still a steinkern.
As Grabau says, there are times when an internal mold exists inside of an external mold, and, through pressure, the external mold is pressed against the internal mold producing a steinkern with the features of both the inside of the shell and the outside on the same object.
As to your question about a source for a definitive definition and why the literature seems to use mold and cast interchangeably, I refer you to the varying opinions we here have all given to you. Even though my answers are the only correct ones 😉 I believe the others are welcome to their own definitions even if they are wrong….. LOL