I'll defer to others on the 6 and 7 speed reference. I thought (may be wrong) that the 'Z' prefix was the going forward name/model version for the 6 and 7 speed but you raise a good question.

Honestly, I find component model numbers are often very confusing (not just KMC but for other manufacturers, too) for someone like me who is trying to get into my own bike maintenance and is trying to figure out what components to buy and what makes a difference in terms of quality and durability. I sometimes will see slight differences in the labels/models on sites like Amazon but it's not always clear if it's just how the seller is labeling it or if there's really a difference. I'm a bit of a cheapskate so I look for price but I don't want to sacrifice obvious quality differences to save a few dollars in the short term. It's probably all quite simple if you're an experienced mechanic but not so much for someone like me .