This list all looks like it came from Stampeaz. The responses you have gotten already have been pretty spot on, but here's the official list.

1. Mystery C - was a mistake at the factory that came out rubbery and I thought it was difficult to carve on, which was why I graded it as "C" and sold it for really cheap. I used some of it as simple background stamps that didn't require much detail and that was pretty much all it was good for.
2. NZ Kut, as people have noted was the iteration before OZ. A little more elastic than OZ, but still takes good detail. If you sand it before doing your transfer, it will take both the transfer and the printing ink better.
3. B sounds like an old block of PZ Kut. I believe I always identified which material I was grading after PZ Kut went away, so OZ would have been marked OZ-B.
4. OZ still had a little elasticity and release issues, but held detail well and, if sanded before you do your transfer should hold ink without much difficulty.
5. Really shiny on both sides sounds like Firm Kut. It was my first try at a new formulation after PZ Kut was wrenched from my little webbed feet. It's pretty firm, definitely requires sanding before use and needs some patience to work with, but I've been able to do some amazingly detailed carvings with it.

Currently, I've got Tan-Z Kut. It's still a bit firmer than PZ Kut, but not as elastic as the other tries I've made at reformulation, so cuts release better. I like it pretty well and it seems to do well with most inks. I still always sand before transfer, but I think that helps any block. If you want to look at responses from my various testers when I was developing the stuff, the final tests start on my (recently much neglected) blog here: Tan-Z Sample Results Round 1. There are several other posts after that in fairly quick succession, if you want to see some amazing carvings and read about the various tests it was put through.

I hope that's helpful. :)

Webfoot

Thank you Webfoot! Very helpful! Yes, they did all come from Stampeaz......I hope to try some of that Tan-Z cut soon :)

QM

I see that there is still some Gray-Z still left. It is thin enough to work for micro-postals. When you make another run of Tan-Z it would be useful to have some material as thin as Gray-Z.

I see that there is still some Gray-Z still left. It is thin enough to work for micro-postals. When you make another run of Tan-Z it would be useful to have some material as thin as Gray-Z.

Duly noted. I will see if I can get a little bit in the half thickness next time around, without having to buy a full ton and a half of it. :)