I don't recall any Phase I , or Phase II cars.
I think that ad is referencing Phase I , Phase II , and Phase III engine options which is in some of the ads, but I don't think those were used very often either.
In the second part of this interview Joel Rosen talked about the “Phase-III” name.
http://www.corvettereport.com/the-mo...ience-part-ii/
quote from this interview....
"Being a Corvette guy right from the beginning, that’s where my roots were, so now, I had an “in” with Baldwin Chevrolet. I went to them and talked them into doing a package, which was our SS-427 basic car and we went on.
Part of the name was that because we used our Phase III Ignition system, we decided to call the cars “Phase-III.” We also had our own Grant steering wheel business, which very few people knew. The capacitive discharge ignition system we had built for us, we called it a Phase-III Ignition. "