The K-27's were built in 1903 as Vauclain Compounds. In 1905 it was decided to convert one locomotive to simple as an experiment. No. 458 was converted to simple and slide valves in 1906. As a result, the other fourteen were converted to simple and slide valves, between 1907 and 1912 (456 may have been converted as late as 1915).
In 1923 it was decided to upgrade the K-27's to piston valves and presumably superheat. Beginning in 1924, the first four (No's. 454, 456, 458 and 461) were rebuilt with inside admission piston valves, allowing the use of the original Stephenson valve gear. These cylinders slope in so the valves would line up with the inside Stephenson gear.
Between 1925 and 1929, the next batch (No's. 452, 453, 455, 459, 462, 463 and 464) were rebuilt with outside admission valves and Walshaerts Valve Gear. These cylinders slope out, to line the valves up with the valve gear. The orginal four rebuilds also received Walshaerts gear at about this same time, but kept the inside admission valves. Rocker arms carry the valve motion from the outside Walshaerts gear to the inside located piston valves.
No's. 450, 451, 457 and 460 were not converted to piston valves.
This information is from Dennis O'Berry's excellent book, "The Mudhens."