On reviewing the photographs, I realized this is a very unusual swing bridge. There is no ring gear on the center pier, and at the shore end of the bridge, there is a ship's capstan and fairlead pulleys! It appears that to save money, this bridge was designed to be winched open and shut using chains or cables, as opposed to the conventional geared drive.
This is a non-symmetrical swing bridge; there is a counterweight on the shore end to compensate for the difference in span length.
(On edit, from the book "Movable Bridge Engineering":)
"There are some examples of bobtail or asymmetrical swing bridges, trussed type and girder type, from the early twentieth Century, but they seem to be concentrated in the upper Midwest. The Chicago, Milwaukee & St. Paul Railroad was a particularly vigorous proponent of this type of bridge... and at rural locations, such as the Wisconsin River. Most of these bridges are still in use for railway traffic, but some swing bridges in rural areas no longer open for navigation. ...but some examples, such as the one over the Wisconsin River at Sauk City, Wisconsin, is combined with several truss-type approach spans. The necessity of adding an expensive... counterweight to balance this bridge, when a symmetrical second arm could easily been added and the approach spans shortened, seems incongruous. It appears that the bridge was rope-operated from shore, so that the short shoreward span provided some advantage in the shorter length of operating rope required and a better angle of incidence of the tensioned rope at the span."
The swing span of the bridge was originally constructed in 1901 by American Bridge Company (Lassig Branch). In 1908, the through girder spans were placed along with a 216' secondhand Whipple Through Truss. In 1943, the Whipple Truss was replaced with a pair of secondhand pony trusses, a 112' Span built 1899 at Oxford Junction, Iowa and a 96' Span built at an unknown location.