Thanks, found it! It's a "mortise chisel and drill": a much more practical solution that I can buy!

While the Reuleaux concept is mathematically cute, it isn't very practical and is only approximate. And the Reuleaux bit requires a special chuck to ensure that the square is "upright" (not tipped at some arbitrary angle): http://upper.us.edu/faculty/smith/reuleaux.htm for details.

There's a joke in this somewhere about mathematicians and carpenters.