The earliest Model 1857 movements manufactured by Waltham feature a “sprung-under” hairspring and balance. In this arrangement, the hairspring is fitted underneath the balance wheel, and the balance cock can be removed independently from the balance.
In early 1860, the company introduced a new “sprung-over” arrangement, primarily implemented on the Appleton, Tracy & Co. grade movements until 1868. The sprung over design provided more clearance for the hairspring and made adjustments much easier. The new arrangement also allowed the hairspring stud to be mounted to the balance cock, a design pattern eventually adopted as an industry standard.
The sprung-under movements can be easily identified by the regulator being mounted on the upper plate under the balance instead of originating from the balance cock.