What I didn't know was when the discovery was made.
The below release from NARA is dated in December of 2005.
The records I retrieved from Ancestry.com with the wrong occupations - I retrieved from May to October, 2007.
I still don't know when Ancestry's records were updated.
The following comes from the "Reference Copy of Technical Documentation for Accessioned Electronic Records." (PDF available from NARA.gov)
Supplemental User Note 5
Army Serial Number Electronic File, ca. 1938 -1946: Merged Processed File NN3-064-03-008
Civilian Occupation Codes
Following numerous user complaints that the Civilian Occupation Code interpretations did not match what was known from other sources, an investigation was conducted into the sources of the code books and keypunch operator manuals used when punching the Enlistment card's Civilian Occupation Code field.
There were several code books and operator's manuals in use during Wodd War II for coding civilian occupations, depending upon which War Department form or IBM punch card was being punched. Two codebooks were discovered in the records of the Textual Records Division, Modem Military Records (NWCTM) and Modem Civilian Records (NWCTC): Civilian Occupational Classification of Enlisted Personnel (TM 12-426) (War Department 1 July 1944) and Army Regulations No. 615-26 (AR 615-26) (War Department September 15, 1942). These employed two civilian occupation coding systems, the Dictionary of Occupational Titles (DOT) and the US Army's Specification Serial Number. The latter was considered by Army to be a better way to match soldiers' civilian occupations with their military occupation specialties (MOS). Both were used when keypunching W.D., A.G.O. Form No. 20, the Soldier's Qualification Card. Both coding systems were mistakenly used together when assigning interpretations to the Civilian Occupation Codes found in the Army Serial Number Electronic File, ca. 1938 -1946: Merged Processed File. When used together, the two coding systems frequently assigned two different interpretations to the same code, resulting in user confusion.
Civilian occupation information was furnished by the enlistee or inductee during their interview at the Recruit Reception Center. Based upon the individual's statements, the interviewer wrote down an Occupation Code that was looked up in was looked up in Code No. 30, Civilian Occupation, in Technical Manual 12-310 (15 January 1944). Code 30 was supplemented by Code 30A, Dictionary of Occupational Titles (DOT). This code written in a box on the individual's Enlistment Record (W.D.A.G.O. Form 22) or the Report of Physical Examination and Induction (Armed Forces' Original D.S.S. Form 221). Though no complete version of Code 30A was found during our investigation of the coding errors, three pages were found in a draft version of Technical Manual (TM) 12-310 that was circulated on July 7, 1942. Later (1945) WD AGO Form 372 keypunch operators were instructed by the Machine Records Operation Technical Manual (TM 12-305), 1 November 1945, that ''the first three digits ofthe codes in the Dictionary of Occupational Titles will be used."
Upon discovery that Civilian Occupational Classification of Enlisted Personnel (TM 12-426) (War Department 1 July 1944) and Army Regulations No. 615-26 (AR 615-26) (War Department September 15, 1942) were intended for use with the Soldier's Qualification Card, these folders were removed from the documentation. They were replaced by copies of portions the Dictionary of Occupational Titles, Part I of III, Definitions of Titles, Prepared by the Job Analysis and Information Section Division of Standards and Research (Washington, DC: Government Printing Office, 1939, 1943 and 1945).
Accession No: NN3-064-03-008
Prepared by Lee A. Gladwin, Archivist
Date: December 28, 2005