I do know that a lot of the C17s had "shotgun" smokestacks, And I belive somewhere I have seen an early picture of 315 with a normal tapered stack, could be wrong though. Something could have happened to the origional to cause her to "borrow" one from another loco (a c-17 perhaps). I do not know either way why it was added (if it was), or why it was removed.
I belive it was the late 40s that the straight stack was removed.
Casey
PS Cant wait to see her run, this is the third summer in a row I will go to see her, maybe Ill get my chance this year...