Using larger cards for main headings as Miles suggests (1905/1899) is very similar to using tabbed dividers. When were these invented for separating groups?

The original tags from antiquity did this sort of functionality as they stuck out from the shelves as a finding aid.

While the today / next / someday cards are labeled, they're also color coded (white, light tan, tan) to help distinguish them.

Though not defined in the Analog system, these differently colored cards could also be used to indicate different sorts of data, though one would need to potentially ignore the pre-printed labels of "Today", "Next", "Someday".