coin18889, as a general rule a "Z" prefix indicates a replacement--but not always. I did not find your note in the
Mehilba World Replacement book nor is there any reference to a replacement in the
SCWPM General Issues, page 1067. Doesn't mean it isn't a replacement, so maybe someone more knowledgeable than I will post.
With all that said it is a NICE find! And pardon my lack of manners for not initially saying:

to the CCF.