Someday scientific methods will be able to pull DNA from beneath stamps, or the gum of the envelopes, run it through a database such as those that can tell a person's ancestry from their DNA, and from there they'll be able to zero in on the identity of Z. We're basically waiting for advancements in DNA identification, while hoping LE has preserved samples that will nail Z.
I sometimes think about Thomas Jefferson, and his great secret (that he fathered several of his slaves himself, by Sally Hemings.) It was much suspected and talked about while he was alive, but he never admitted it, nor did Sally that we know. One or two of the children, Madison Hemings for one, claimed to have been fathered by Jefferson, but there was no proof. Then, over 150 years after Jefferson died, DNA proved he was, in fact, the father of those kids.
The same type of thing will happen with Z, I believe. He may have been "crack proof" in his day, but science will catch up with him eventually.