Below, the detail shows the air-box design on the rear of the ashpan.
The idea behind the air-box design is that, in order to escape through the dampers, the sparks will not only have to defy gravity and negotiate a more convoluted path to escape, but do this against the flow of air being drawn down into the ashpan.
In the photo above, the rear air-box is tack welded in position (to the left of the photo), and the partially fabricated front air-box is resting on top of the ashpan (top right).
