I saw this interesting question a couple years ago, but there was no clearcut answer or conclusion. I would speculate that at one time each was hand crafted, but I can't imagine in this age of digital everything that would still be the case.
It's definitely a unique style.
A filter called "India Ink" from
www.FlamingPear.com was used on this tongue in cheek example. It's in the ballpark, but I'm sure this is not what's used. A filter that renders similar effects, "Engraver" is available from
www.Panopticum.com.
~Danny~