There are some online pieces about Twain's pens at

http://www.kamakurapens.com/TwainsConklin.html
http://www.kamakurapens.com/TwainsMackinnon.html
http://www.kamakurapens.com/TwainsEarlyImpliments.html
http://www.kamakurapens.com/TwainsWirt.html
http://www.kamakurapens.com/TwainsWaterman.html

(some are still "under construction")

Pat Ober