Yes, agreed. So far of the five complete K4's I've owned, three were early frames and two were the later frames. I've gotten the reverse switch in the stack on a couple of them as well including an earlier frame version. Both my SP Mountains have the reverse switch in the stack. It is amazing to ponder just how many K4's Scale craft may have produced over that lifespan. I know Bob has owned plenty of them and he doesn't even focus on Scale craft. I am sure you have more than a handful and I have four now with my eyes open for more of them for some reason.
Most of the deals I find on them is because they are not advertised even remotely correctly and I have a keen eye for searching. Heck I once found a spare CLW H10 complete frame in a pile of junk listed under American Flyer S gauge. I just happened to spy it and knew what it was because I knew I needed one!! Knowledge is key then comes timing and luck lol
I'll add, the one piece frame may have come a bit earlier as my unbuilt K4 did not have a pre assembled boiler but did have what I think was a factory assembled chassis. So assuming my loco "kit" actually left the factory together originally, that might suggest the one piece frame started before 1938 when they advertised the pre assembled boiler. Then again, they could have still offered the craftsman kits lol who knows. 
All I know is I like K4's!!
Below, later-ish one piece frame I think factory assembled but without factory built up boiler. Probably a good thing looking at that tender!!
This one might be the ugliest one ever built. I am not sure what the builder was thinking when he did the boiler. The tender is pretty crude as well. It's really more tinplate-ish than scale-ish.... part of the reason I like it lol Three piece frame on this one.
This is the one I sent to Daniel in France, it was an early three piece frame as well.
