The botton line is that the rollers always have some sparks on all of the post war locomotives; and,
Worn rollers spark a lot. Your rollers, while nominally functional, have a lot of miles on them and need to be replaced. I've seen this problem on many of the early F3s (growlers) , but not so much with steamers.
You my wish have your LHS or a seasoned "post war stuff" friend do this one. Watch and learn, for the next time. Changing rollers not a repair for the faint of heart, especially the first time you do it.
Welcome to the world of post war 1950's stuff. It's a whole different adventure, but lots of fun.