From The Industries of Saint Louis by J.W. Leonard (1887) is this drawing of the Republican Building, a fine Second Empire built at 3rd and Chestnut in 1873 (two blocks north of the Old Cathedral), after a disastrous fire in 1870 destroyed the previous building. The designers were Thomas Walsh and Edward Jungenfeld, who incorporated a cast iron facade on the lower two stories and hydraulic pressed brick on the upper three stories. A festival and speechmaking marked the opening of the building in January 1873. As a nod to the fate of the previous Republican Building, the 1873 form had several fireproofing options -- iron ceilings, cement floors (covered in pine), fire hoses on every floor, and water tanks were kept on the premises.
It appears the fireproofing lasted until the Jefferson National Expansion Memorial demolitions.