Mystery Part Copper/Bronze Stove Furnace Shelf Threshold Boat Door 297

An antique mystery part for wood/coal stove? Cast copper or bronze - hard to tell. This was in a dealers estate of salvaged stove and architectural parts. I have no idea what product this "shelf" fits.

Embossed on bottom 297.

Size 18" x 5.5" at widest... weight 4.5 lbs.