large antique vintage steel architectural floor register grate for heating, cooling and ventilation. This doesn't have any louvers or flow controls. It's just a standard steel grate.
It measures about 23 1/2" x 11 1/2" inside (as shown in the 5th photo) with an about 1 1/4" flange on the ends and a 1 1/8" flange on the sides for 26" x 13 3/4" overall.
This has a patina of age and rust.