Cane Creek Crosstop OS Auxiliary "Cross" Levers f/31.8mm Drop Bars. Black
Initially marketed primarily for cyclocross, these are also an excellent choice for the touring or long distance cyclist, permitting you to brake from the top part of the handlebar. Unlike the 1970s extension levers, these don't interfere with the main brake levers. They install in the middle of the cable run.
These are design to fit the "new oversized" 31.8mm handlerbars.