On the other hand, some of the variations are significant, even now.  The first patent may have been for a design that was difficult to machine; or awkward to use; or quickly wore out.  The second patent introduced a new way that not only got by the first patent, but also produced a better article of commerce.  Never mind that the mechanism's basic operation was the same.  My best guess is that there were so many unpatented mechanisms in common use (in the public domain) that if the concept was unpatentable, the execution was.  Witness the little roller that keeps the bevel gears of a No.2 Millers Falls eggbeater drill in near-perfect mesh on the pitch lines of the mating gears.  No one ever found a better way; yet the only patent which illustrates this mechanism (that I have been able to find) uses an instantly recognizable rendition of that model as an example of the quintessential geared bit driver: George L. Wilcox, "Locking Device for Hand Drills" US Patent No. 1,083,784, January 6, 1914 - "It is understood that the hand drill so far described is of the standard type of drill now generally in use." Mr. Wilcox's witnesses were Theo. G. Hoster and Philip D. Rollhaus.

Note: Until this study is fully published, you can link to the brace images by copying the file name of a brace (in the column at left in the table below) and then pasting it onto the end of this URL: http://www.georgesbasement.com/braces/