Yeh, I was thinking about the same. My intention was to get out the 1921 roundhouse fire photos and see if I/we can figure out which wood cabbed engine was not there, which would make it a candidate for the one in this photo. That is assuming all of the wood cabbed engines were rebuilt with steel cabs which might not be correct. I think the story was that the 3 Baldwin Mikes and #50 were all at Austin at the time of the 1921 fire.
I assumed the photos of a gleaming freshly painted #4 were after the 1921 fire, but it could be as you suggest without further info.