Selecting the most accurately dated AVS carb casting

As long as it's prior to the build date in that model year, nobody could say anything regarding the date.

You could have a carb on it that was dated sept 68 as long as it's the correct numbered carb. General observations in most parts is 2-4 months prior. "Just in time" inventory procedures were not in use back then. I've seen cars that had a SPD that was 3 weeks before the engine was built... yeah, that doesn't exactly jive.

The SPD is only a scheduled production date, not the actual.