The links below are zip files containing PDF's of IRS Forms 990 (with Schedule A), 990-EZ, and 990-PF from 1992-2004 with corresponding SOI field names identified directly on each form for all of the SOI files on the NCCS DataWeb.
These PDF's and standard data dictionaries are provided by the IRS each year. While IRS forms rarely change, the PDF's are arguably the best way to identify SOI variables over time, as SOI variable names are generally not very descriptive (e.g. - E010, E020, X120, etc.) and SOI naming conventions have changed over time.
Example 1: SOI has used the same variable name for different fields on different forms within a single year. The SOI 2004 files use R030 for Form 990, Line 1c (Government Contributions) and also use R030 for Form 990PF, Line 4, column (a) (Dividends and interest from securities).
Example 2: SOI has reused variable names for different fields within the same form. Form 990 data files prior to 2000 used E010 for Zip Code and E040 for Line 9a (Special Events Gross Revenue). Beginning in 2000, Form 990 data files use E010 for Tax Year Ending Date and E040 for Zip Code.