Alex,
PRR, K4s #5399 was built at the railroad's Juniata Shops in August of 1924 as their construction number 3924. It was built as a conventional class K4s locomotive. That information is reflected on your builders plate.
In the Summer 1939 the Lima Locomotive Co. fitted 5399 with the Franklin system of steam distribution with poppet valves actuated by an eccentric crank arm with the actuating machinery located on her pilot deck between and in front of the cylinders. Shortly thereafter PRR had Lima apply a front end throttle system to this locomotive. These experimental modifications produced 44% more drawbar horsepower at 80 MPH. Two other K4s locomotives were fitted with experimental poppet valve assemblies. In 1940 5436 received a Jones steam actuated system and in 1945, the 3847 received the Franklin type B rotary cam poppet valve system. These experiments were a tremendous direct influence on the design of the class T1 passenger engine, all of which were built with poppet valves.
The three (3) poppet valve K4s locomotives were unofficially classed K4sb, K4sc and K4sd, although were never so formally listed in the form 109 specification book, or on the engine badge plates. The 5399 was retired in November of 1953, 5436 in November of 1955 and 3847 in November of 1953. Not proven, but is assumed, that the engines met their end with their poppet assemblies still intact.
It would probably safe to say that the plate you have is the original, as we know the railroad never officially changed the class of the locomotive, even after all the experimental modifications had been made.
Hope this helps.
Buzz
