This series may have irregularities or important features that are not disclosed here. To see whether this is the case, please consult Part 1, Section 1, Table 11 in the original source at https://fraser.stlouisfed.org/scribd/?item_id=6408&filepath=%2Fdocs%2Fpublications%2Fbms%2F1914-1941%2FBMS14-41_complete.pdf&start_page=1
Relevant details can be found in the footnotes of each table as well as the introductory material for Section 1.
Records for 1940 and 1941 rely upon 1940 census data, whereas records for 1936-1939 rely upon 1930 census data.
Board of Governors of the Federal Reserve System (US), Time Deposits in Member Banks in Places with Fewer Than 15,000 Residents [X11TDO15L], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/X11TDO15L, May 21, 2018.