I am rebuilding the original defiant 1piece fireback to a 2 piece fireback. It came with the house I purchased. While following disassembly instructions it mentions a smoke shelf under the damper. Mine did not have a smoke shelf. What exactly does this smoke shelf do? Is it needed to run the stove? I found that the original shelf part is now obsolete so I can't replace it easily. Is there an alternative shelf I can use? Or is there any place that might have this obsolete part? Thx!