All the 125 class was converted to simple engines with slide valves and Stephenson Valve gear starting around 1908.
Around 1918, 454, 456, 458 & 461 were rebuilt with piston valves, retaining Stephenson Valve gear.
Starting in 1925, 452, 453, 455, 459, 463 & 463 were rebuilt with piston valves Walschaert Valve gear and the boilers were superheated.
Somewhere in this same time period 454, 456, 458 & 461 received Walschaert Valve gear and superheating.
In 1929 462 was rebuilt with piston valves and Walschaert Valve gear. Boiler was not superheated.
450, 451, 457 & 460 were never up graded and operated with slide valves and a saturated boiler until retirement - officially 1939, but some had not operated on several years.