What's the difference?

StrokerScamp said

EGR came out in 1974. The EGR ports would have had to be the 1973 model year only..

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You seem to be contradicting yourself. If EGR came out in 74, then "EGR ports" in 73 would STILL have been "EGR," and furthermore, the diagram I posted was 72

It might well be that Cal eee foooornia had EGR before the U.S.