Panic....I've looked at a ton of GMC heads and own more than most guys. In my opinion your cut away appear to be one of an exhaust port. My reason is the divider and a shadowed area behind it and the left edge appears to bet retangular in shape and not circular. The valve area is very small and the chamber appears to be one of the last small port ones made with the shrouded intate and exhaust. Hey I could be wrong and often times I am. JD

216.158 MPH 12-Port 302 GMC on 70% 171.0 MPH 302 stock head on gasoline 7 years later