I noticed this table on Wikipedia (link) and was struck by the top left corner & just how much information was conveyed by this small change. I’m guessing the bulk of this was computer printed and some extra lines ruled onto the page because of the extra line.
If I was doing this today, I would use spreadsheets that simply don’t have this capability, diagonal lines aren’t easy to use in cells. I could fit the information in by making the font smaller in the header row but the outcome would be less readable than what has been done.
Anyway, I just liked the way this was done.