Jim,
A great question and I have to answer that I don't know. i suspect that you aren't getting any other input in that no one else is sure either. There is no reason that CS smooth sided Read shells couldn't have been made like the Parrott shells with lead balls drilled out for the powder chamber and so no need for a side plug. But that said, I've not ever seen a rifled case shell with lead balls that I can remember. As you say the books don't show one. So perhaps the Confederates only used case shot in balls in rifled shells until later and they opted for iron balls using the side loader holes.