Via Tomalak’s Realm, I found this history of the tab at Technology Review, which gives a bit of background on card catalogs as part of the article “Keeping Tabs.”
The tab was the idea of a young man named James Newton Gunn (1867š1927), who started using file cards to achieve savings in cost accounting while working for a manufacturer of portable forges. After further experience as a railroad cashier, Gunn developed a new way to access the contents of a set of index cards, separating them with other cards distinguished by projections marked with letters of the alphabet, dates, or other information.
Apparently Gunn also went on to help found the Harvard Business School… which is a more lasting legacy?