The Leyden Hearth was one of the first popular fireplace inserts. The unit consisted of an inner chamber which was installed into an existing masonry fireplace and piped up to the Damper
This box contained a grate and glass doors, which helped to limit the combustion air intake.
The unit had no outer wall, but instead used the space between the metal firebox and the wall of the existing fireplace as the area to circulate convection room air.
A close copy of this design called Fuego has been marketed over the last 3 decades.
Although more efficient than an open fireplace, these designs do not equal the efficiency of a modern EPA Approved fireplace insert.
Old Ad for Leyden Hearth: