We find, however, that John Payne and Major Hanbury rolled sheet iron as early as 1728 at Pontypool, and patents were granted to other Englishmen before Cort's day. -- from The ABC of Iron by Chas. W. Sisson, 1893 (page 91)
Sheet metal had been available for a long time, but in stove manufacture stoves were overwhelmingly constructed out of cast iron until sometime in the second half of the 19th century. For a number of reasons, at the beginning of the 20th century more stoves included sheet metal parts and more parts were fashioned from sheet metal. By the end of the 1920s, sheet metal use was overtaking cast iron. About this time fireboxes made of sheet metal for open fireplaces began being marketed. Some were constructed and welded on site, which usually vented to a masonry chimney. I have seen these welded types in some homes constructed in the 1970s. Other sheet metal type open fireplaces were preassembled by the manufacturer, referred to as 'factory built' and usually vented to metal chimney pipe. The advantage was less cost, and often included a 'modified' feature for a duct to heat room air.
This 1960 house, with a garage converted to living space, has a factory built fireplace with a brick facade. No modified features are evident.
This home, built in 1988, has a firewood burning factory built (metal firebox) open fireplace angled on the corner of the room, that has the 'modified' feature built into the factory built unit that allows room air to enter the lower register, travel below then behind then above the metal firebox to be heated and return to the room through the top register. This open fireplace does not appear to have any outside air ducted to it, but it does have builtin ceramic glass doors. The brick surround is decorative only, as you can see the chimney is metal above the roof. Many modern builtin gas burning fireplaces look nearly identical to this example, including the duct to heat room air, except they often have faux logs and smaller diameter metal chimneys.
This firewood burning factory built (metal firebox) open fireplace angled is located on the interior space of the floorplan. I was not able to determine if this unit has any modified features. You can see from the exterior view it has a metal chimney flue.