Baltimore City 1940 Census shows last residence in 1935 being rural in Baltimore County, with the residence code filled in later as "7251." Where can I find out what that code means?
This is a preliminary answer that will be added to as I find more information -- see the area below the dividing line for updates. The short answer is that the code 7251 is the designation for Baltimore county -- it is equivalent to the information in the fields that come before it. These codes were added in the office by the workers preparing the statistics from the census, and were supposed to encode the same information that the enumerator wrote.
You can find the Codebook for the 1940 Census at the website of IPUMS-USA: (Minnesota Population Center's Integrated Public Use Microdata Series projects -- IPUMS). Page 37 of the PDF (page 2.5) says:
Place of residence in 1935 was coded by comparing the alphabetic entries of 1935 place, county and State codes (columns 17-19) with files containing names of all places with populations greater than 2,500, counties and States. The resulting numeric codes were compared with the 1940 census code in column D to check the accuracy of the machine coding procedure. The column D code was also used when the alphabetic entries in columns 17-19 did not provide sufficient information to code 1935 place of residence.
To decipher the codes in column D, see page 366 of the PDF, under "Instructions for Coding Migration". The enumerators were instructed to reference a group of 7 code lists that were used for the 1930 census (see page 370 of the PDF -- it is an image, so I haven't copied it here). These refer to the MCDs (Minor Civil Divisions) used for 1930. I haven't found the lists yet, but if I can find them, I'll add the links here. Some information about "census geography" can be found on NARA's 1930 Census page Part 2: Indexes and Other Finding Aids.
For an explanation of MCDs, see Prologue Magazine, Fall 2003, Vol. 35, No. 3, Genealogy Notes: "Plans of Division": Describing the Enumeration Districts of the 1930 Census By Claire Prechtel-Kluskens.
There are many One-Step Webpages by Stephen P. Morse which relate to finding census records, and the various FAQs on how to use the site can yield many clues about where to find resources.
One of the One-Step Web Pages is Deciphering Miscellaneous Codes Appended to the 1940 Census in One Step which refers the user to a procedural history of the Census by Robert Jenkins, also available for download at IPUMS. However, a quick glance at the table of contents suggests that this will be useful mostly for coding for occupation.
- 1940 Census instructions to enumerators (From archives.gov 1940 Census site)
- Questions Asked on the 1940 Census
- Enumeration forms index at IPUMS
- 1940 Census Records online research guide at US National Archives
- Wikipedia: Census Designated Place
Update to be added later about deciphering the specific code 7251 in the question.