Smokey Yunick mentions developing the head in the late 60's. When static compression gets much over 11.0:1 the standard plug location can become drenched with fuel. So Smokey would plug it and re-machine an angled mouting to move the plug. After several years he finally convinced Chevrolet to do it. I believe the '492' head did not have a passage for manifold heat. My 1975 Chevy Power manual mentions the machining of the casting was changed in 1972 to accomodate larger valve springs.
As with many of his ideas, someone else got the credit.