It's literally the box the exhausts, heat exchanger and muffler all connect. Box rusted a little and some rivets failed. My dealer said the HM rep said they stopped making that part, but could manufacture me one in a month if I'm lucky. Said they'd send my dealer the plans and they could make one but im sure that's $$$.
I just disconnected everything, took it out and ghetto rehabbed it
got it to stop flapping and I'll run it until winter.
I'm going to call hydramaster and see if they would send me the plans. If they do or don't I'll make one this winter. It's literally a box with some holes in it.