small block w-2 heads

I have some articles on the Arrow and W-2 stuff from when it was introduced buried here somewhere at home. I don't think that the W-2 heads were anything special unless they were supplied not "finish machined". Glidden reconfigured anything that he thought would help, including what I believe involved cutting out the lifter valley and rewelding everything back to correct the "inefficient" OEM valve train geometry in small blocks.

Some stuff I remember reading about Don Carlton's testing of the small block was that the OEM blocks were not up to the HP being generated. Carlton died in a racing accident in the summer of '77, I believe. When did the original X-block come out?