From what I've been able to figure out (based on where I've seen them), the "Z" bags are styles that Coach made in the past, sometimes MFF and sometimes FP. If they have extra fabric or if the styles are popular or if there's anotehr reason, they make them with Z in front of the style number for the discounters like Nordstrom Rack, TJ Maxx, and the like.
I've never seen Z bags (with Z in front of the style number) in full priced department stores, whereas I have seen E and M bags (with E and M after the style number) in department stores.
My quoting skills are subpar, but I wanted to thank both of you for such quick and detailled responses! I found an "X" within the black interior tag - does this mean the same thing as the "Z?"