alt Hacker Newshttps://archive.org/details/Programming_the_Z-80_2nd_Edition... is where I learned about the "W" and "Z" registers -- if you don't know what they do, I won't spoil your fun reading Zaks! See also http://www.z80.info/zip/z80-documented.pdf
The MOS 6502 was introduced at the WESCON tradeshow in September 1975 and sold for $25 quantity 1. They had a transparent vase full of them, 'proving' they did volume -- but those were all chips that failed post-manufacturing testing, except for the very top layer. Still...
Spoiler: the undocumented WZ registers are just microcode temporaries to hold large operands. For instance when reading a jump instruction, the jump destination can't be read directly into PC since that would mess up reading the second half of it, so it's read into WZ and then WZ is transferred to PC. This is invisible to the programmer.
One might say the instruction is really "jump to WZ", or JWZ, which, of course, refers to Jamie Zawinski (not really).