Z or 645MkII, or whatever.
I see it as continuation of Ricoh's primary school exercises in branding: let's name the company Pentax Ricoh Imaging Corporation, but no, no, let's change it, better shorten it, to Ricoh Imaging 12 months later.
Same 645 MkII, MK2, or 645 2014. When you replace whatever comes after the 645 with an unknown variable x to define it finally, it looks like 645x. But X is already been used throughout entire industry, so let's replace the unknown variable with a "y", instead? But nah, it sounds weird, because y is pronounced as "why", and people can start asking weird questions: 645 why?
So let's forget it, and use the third letter often used for unknown variables in equations, and that is "Z".
Don't put much thought into it. Leica's X substantially means nothing, and overlaps with half a dozen of other Xs on the market, but they're trying to make quite an iconography about it.
Very few companies come with branding that redefines something.