Is it possible that the early spark arrestor stacks on 19 and 20 as shown in the builders photos and the Ruston stacks, simply fit over the top of the straight stacks we have today? I've noticed that the present straight stack on 19 has a lot of pitting in the cast iron like it has been subjected to a harsher enviroment than simply out in the weather. I doubt if it was the salt air up in Skagway as this pitting is not present elsewhere on 19. Regardless of whether or not the spark arrestor stacks fitted over the straight ones, is it possible that the spark arrestor stacks were removed during the winter, wet weather or otherwise times of zero fire danger. Considering that they were burning mill waste that was sometimes less than ideal fuel, the spark arrestor may have required freqent cleaning to keep them from clogging up, so it may have been desirable to run without them when practical.