I saw these while browsing through a thrift store and correctly clocked them as thick tabbed dividers though the person who rang them up for $1.50 thought they were folders because of their heft. 

I’ve been wanting some for use with 8.5 x 11″ index cards, but thought that most major stationers had quit manufacturing them decades ago. It turns out you can still get them from Tops via a variety of different office supply outlets, though closer to the range of $15-40 per pack. I’m sure these will work wonders in my Shaw-Walker filing cabinet. 

Interestingly, one outlet I saw was pitching them for dividing files within a single a folder, but I’m guessing they weren’t aware of how these were used historically. (The entire group of 25 would fill up a standard manilla folder all by itself!) It’s sort of a shame that they only count up to 25 instead of 31 so that one could also easily use them as a days-of-the-month tickler file.