I've seen one picture with 464 after being converted to Walschaerts, lead by 456 still with the Stevenson valve gear and piston valves (the inward canted cylinders). It is a shot taken in the winter, so it is a little rough and grainy. I believe the location is somewhere near Marshall Pass. One thing to note, the engines are labeled for the Denver & Rio Grande Western, so it has to be after '24. Also, while 464 looks much like she does today, 456 has an air tank on the tender, and none on the engineers side of the loco. I have seen the same pic on three separate occasions, and there are two versions. One is low quality and very hard to see, and it is very possible to miss the rarity of the pic. The other is not great, but much easier to see, and if you are looking for it, very easy to spot the anomaly...
I do not know who owns the rights to this picture, so I don't know if it will soon be posted here...
Casey