My research has suggested that the bombsight has been known by several monikers; The 20 cent bombsight, The Greening bombsight, and The Mark Twain.
Developed by C.R. Greening, one of the Doolittle Raiders and a remarkable man, the mechanism was cobbled together at Elgin AAFB from scrap aluminum. The Norden unit was ineffective at low altitude and too heavy for the long distance Tokyo mission and as it turned out, the simple device worked quite well at the low altitudes the Raiders would be flying.
The prevailing theories on the "Mark Twain" name are; 1) The Greening family was related to the Clemmons family, so he chose the pen name of his famous relative. 2) In the early days of river piloting, depth was measured by a weighted line with knots at specified intervals. When the the water reached the second knot, the leadsman's call was, "Mark Twain"! The Raider's felt that they would be flying and using the bombsight at about the same height off the ground, hence "Mark Twain"! You get to decide which reason is better!
On a different note, Elmira New York was where the Norden Bombsights were built for a while.