Why is it that in each year’s airframe production of the 19/23/24, there is at least one serial number airframe that was moved from that year’s production range, and instead included in a subsequent year’s production?
I can’t tell you specifically (for your airframe) why the model year was different. I can describe what I have been told (by engineers who worked at the Liberal KS facility where our birds were built), about why some specific serial numbers were shifted to a subsequent model year. There were a number of potential reasons.
The parts books changed as of 1978, for the 19/23/24. If the plant put on one or more standard parts that were unique to the later parts books, the model year was adjusted.
If the plane was being built for a customer who learned of a pending option and ordered it, and that delayed the build, the model year could have been adjusted (but was not always). This usually did occur if the part or option was unique to the 1978 or later year parts books. More often it was just a matter of manufacturing delay related to some unavailable part or option.
The parts books have applicability codes that embrace certain SN ranges. Those ranges take into account the SN airframes that fell into a different grouping.