I see a transistor type package (TO-220?) in the middle that has some rusty look to it.
I assume that's the device that failed.
Question, is the steel bar holding the package down real tight??
It's hard to tell from the pic.. If that device isn't being pressed down hard, that could be why it failed.
TO-220 Transistor Dual Heatsink Clamp
Edit:
Using Zoom, it looks like 1/4 of the ceramic top of the package has
blown clean off.
That thing must have been HOT!
The Clamp seems to be offset a bit on the left, not lined up like the others.