One of my hobbies (my spouse would say "afflictions") is collecting and using vintage fountain pens. In the pursuit of that afflic... er... hobby, I came across this discussion, the central point of which is that Justice Souter is also a fountain pen aficionado; specifically, he uses "an old Esterbrook from forty or fifty years ago."
Fountain pen nuts will immediately know what I mean when I say that I can't think of a more Souter-esque pen than an "Estie." They have a reputation as "blue collar" pens -- no Mont Blancs here -- but while they're not flashy, they are unfailingly reliable. (Full disclosure: My very first fountain pen was a red Esterbrook J, purchased while I was still in graduate school; it lives on my desk to this day).