These Rumford Dampers are very common in masonry fireplaces of solid brick. These are their characteristics:
They have a long rectangular cast iron damper door.
The door has a 16″ long curved cast iron handle sticking down from the center of the damper door. The handle has saw teeth on the back, and a loop on the end.
The shaft of the handle sticks through a metal bracket
The top of the firebox has a tall and tapered throat leading to the damper
The footprint of the firebox is a very sharply angled trapezoid (image 26-3). So the back of the firebox is not as wide as the front.
The nice thing about these Rumford dampers is they have a long taper to the damper and the damper handle is easy to remove. The handle is usually held on with a cotter pin or is screwed into damper handle hinge.
To plug it below the damper, use the Flueblocker
26-4: Where to install a Flueblocker
The Flueblocker can be easily fit into the area below the damper. It seals and insulates very well. You will have to remove the damper handle, or cut a slot in the Flueblocker to accommodate the handle, to fit the Flueblocker into the throat that leads to the damper. When you order a Flueblocker make sure it is at least as big as your largest measurements of the trapezoid. (image 26-4)
What about the Chimney Balloon?
26-5: Chimney Balloon Installed
The Chimney Balloon can work well in this application. It generally installs into the exact same spot the Flueblocker would go. But keep in mind a Chimney Balloon will be about 8″ tall when inflated. (image 26-5)
It gives you a very tight seal, however it is an inflatable. So it is not as durable as the other options. And it requires a top-off on air every 6 to 12 months because of the swings in outside air temperature.