The cleanouts at the rear of the smokebox are there to troubleshoot leaks in the superheaters. With them open, you can hear and (possibly) see if a unit is leaking at the joint with the header, or if the leak is in the unit itself, and where the leak is left to right. Otherwise, you can't narrow anything down until the whole front end is torn out, which is another way of saying that it sucks to be you.
The bolt pattern just above the cleanout is the support for the superheater header. The cleanout is in a very strategic position.