Del, you are correct, after sizing and extracting, that inside neck plug leaves the inside diameter at 0.386. Not at 0.387 for easy bullet insert. The NO 3 die does have inside threads.
Coal, I did read the CH4D website and did not find anything that defined this die. The die code table did not list a NO 3 designation. That and the fact that it has an inside neck sizing stem (which the new dies do not have) is why I believe these are older CH model dies.
I truly was hoping that someone had access to the older CH code tables that defined the NO 3 designation.