A valve gear question

I've been reading this board for some years and though I've only posted maybe a couple of times, I'm very appreciative of the technical information and the wonderful photographs that have been shared. The two threads about the EBT have prompted a question that I've wondered about since visiting the Nevada Northern (NNG) many years ago. Both operating steamers there have Baker valve gear (says so on a plate on the side of either engine). The valve chests, however, are rectangler but quite a bit taller than a standard slide valve chest. The EBT #15 shown in a recent thread seems to have the same setup but with a valve gear I dont't recognise. "Gothpapa's" video of the EBT from the 60's shows the EBT #17 with what looks like the same valve gear connected to a standard piston valve casting. I'm guessing the taller rectagular valve housing is actually a cheaper adaptation to allow piston valves without doing a whole new casting. I'm guessing that all the engines mentioned were given this adaptation to allow super-heating. What do you all think??