Very early Elgin watches are marked "Burt's Patent".
Burt was Merritt Burt (1828-1849) and his invention, later improved by Moseley, protected the watch train in case of breakage of the click, click spring or mainspring; patent No. 44,161 dated September 13, 1864.
From the History of the City of Elgin, published by The Chicago Republican, 1867:
Another advantage will highly recommend itself particularly to the absentminded. Everybody knows that whenever the click-spring of a watch gets out of order, or the click itself breaks, the entire power of the spring acts directly upon the train, and a disaster must occur somewhere, and some of its delicate parts must give way. Most everybody knows, too, that a thoughtless turn too much with the key will very likely break the mainspring, with a like disastrous result to the train. The possibility of these calamities is entirely obviated here by the peculiar construction of the center pinion, which receives its motion from the barrel wheel, and communicates it through the center wheel to the rest of the train.