Progress,  page 101
Here's the finished firebox, and you can more clearly see the curved back and the complex joints
at the back corners it required.  The shelf on either side will hold a heavy piece of angle steel to
support the throat of the fireplace.
I wanted to retain the Rumford design concept of a curved throat to minimize air turbulence at
the front of the fireplace, so I carefully cut a curved edge on 19 1/2 (yeah, that was tricky)
separate pieces of fireblock and cemented them together in a sandwich that spanned the
opening.  The angle steel is 3 inches on a side and 1/4 inch thick.
Here's a side view that better shows the curve of both the throat and the back of the firebox.  
The damper assembly fits exactly between the two.