Trevor, I think you mean in the smokebox, not the firebox. I don't know if screens were ever used in the smokebox of wood fired engines. I kinda doubt it as they could easily plug up with a combination of sticky pitch and soot that would be hard to clean. Screen in the smokebox is sometimes called netting and is more of a coal burner thing I think. What I am refering to is a heavy wire mesh screen on top of the diamond stack that is quite accessable to beat the soot off of. Some wood burners you see had a cylindrical screen with another screen on the top in order to presumably offer more free area in screening to reduce the effects of a partially plugged screen, ie. the screen cylinder made the area of screen much larger than just a flat screen on top. I am no expert as there are a few folks at the SVRy that have much more experience than I throwing wood.