Looking through an old version of Carpentry and Building 1889, I came across this interesting brace, “The Extension Bit-Brace”.
Manufactured by the Diamond Wrench and Tool Company (Portland, Maine), it is described as having thumbscrews, which when loosened allow the head and chuck to move freely along the rod. This allows the brace to be adjusted to have a 8, 10, 12, 14, or 16 inch sweep. Apparently by setting the head for a 16-inch sweep, and the chuck for an 8-inch sweep, a crank motion is obtained doubling the relative torque. It is similar in concept to the Whimble Brace of Goodell-Pratt (No.260-262).
Interesting concept, but I wonder how sturdy the brace is, and how well aligned a hole would be?