A Record Series (otherwise known as Series Number) is how we classify the records or collection.

On the top left hand corner of the Control Card, below the TITLE you can find the SERIES NUMBER otherwise known as Record Series Number.

Example: The Lorado Taft Papers has a Series Number of 26/20/16

26 means the Record Group (Campus Unit), in this case Alumni Association

20 is the Subgroup Alumni

16 is the Record Series Number for Lorado Taft Papers

If you can identify under which campus unit your topic would fall, then it might be easier to search/browse by Record Group:

https://archon.library.illinois.edu/archives/index.php?p=collections/classifications