The valve gear developed at Robert Stephenson’s works was Stephenson valve gear.
It was devised in 1841 by William Howe and William Williams, employees of Robert Stephenson and Company, and named for the firm rather than being invented personally by Robert Stephenson. The design replaced the awkward gab or hook motion used on many early locomotives with a slotted expansion link connected to two eccentric rods.
By shifting the link, the driver could reverse the locomotive and vary the valve cutoff. That allowed the engine to control how long steam entered the cylinders, balancing pulling power against economy. Stephenson gear was widely used on 19th-century locomotives and remained especially common on British engines, where it was often mounted between the frames.
Walschaerts gear eventually became dominant, particularly on larger locomotives, because it could be mounted outside the frames and offered convenient control over a broad range of settings. Baker gear was a later American design, while Joy gear was another separate alternative. Thus, the wording “before Walschaerts gear became widespread” points specifically to Stephenson gear.