A/F/R gauge for tuning

Dale Davies said

That is correct. Now remember that sensor was an input fuel trim to an ECM. This was an early step in better fuel management that was only used for a year or two. Then we had TBI for a bit longer.
What was happening was the overlap allowed raw fuel to escape out the exhaust valve. Whatever happened to the chemistry of the exhaust the fuel displaced O2 at the sensor. This article is from the latter '80's so understanding of electronic controls was not as well understood as today.

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I'm not going to say that is impossible but I think with the information provided that conclusion could be a bit of a leap. Unless you have more data to support that conclusion that wasn't in the post. How was it determined that the motor was lean to begin with? On the first carb did they try unpluging the mixture control solenoid and jetting the carb up to confirm that the motor was lean? Were they using et as their measuring stick for improvement? More details on this test please.