Taken from the webpage of Stephen P Morse....
The following codes have been summarized from the 200+ page 1930 Census Bureau publication "Classified Index of Occupations". These codes were put on the census pages well after the enumeration was done, and therefore provide no additional information. However, sometimes knowing the code can help you determine the occupation in cases where the quality of the microfilm is poor or the handwriting of the enumerator is difficult to read.
http://stevemorse.org/census/ocodes.htm
"These code numbers were not filled in by the
enumerator. They were added later, sometimes years
later, by tabulators checking the census for various
statistics. They were originally written in red pencil
so the tabulators could find them easily and know that
they were not put there by the enumerator, the problem
is that the census pages were microfilmed in black and
white and the originals destroyed. The codes have no
importance to anyone doing research - they were a
simplified way for the tabulators to see how many
people were foreign born, born in different states,
etc. The numbers were created using the information in
the census. Nothing new will be found from these
codes. It's the same as the occupation codes often
found in the last few columns of many census years."
Researcher, Mark Jacobson
Boca Raton, FL
CODES
In addition to the 13 numbered columns, the form # 15-202 contained
three lettered columns "for office use only" into which
a tabulation code was entered:
A-state code
B-country code
C-Nativity
D-occupation code
Subsequently, the data from the schedules were tabulated for statistical
purposes, and this information was incorporated into the Bureau's
published reports on the fifteenth Census.
For a complete list of Nativity codes: see U.S. Department of
Commerce, Bureau of the Census. Coding Instructions for the Population
Schedule. fifteenth Census of the United States, U.S. Government
Printing Office: Washington, DC, 1930, (C3.37/6C64/1-3).
For a complete list of Occupations codes: see U.S. Department
of Commerce, Bureau of the Census. Classified Index of Occupations,
Fifteenth Census of the United States, U.S. government Printing
Office: Washington, DC, 1930, (C3.37/2:Oc1/2-2).