In 1936-37 the St. Louis-San Francisco's Springfield shops erected 34 Mountain types from parts of retired locomotives; the first eleven of these rebuilds, including No. 4307, came from retired 2-10-2s. They had 27x30-inch cylinders, 70-inch Scullin disc drivers, and a boiler pressure of 250 p.s.i. Evaporative heating surface totaled 4720 square feet, superheater surface 1834 square feet, and grate area 70 square feet. Weighing 431,110 pounds, they produced a tractive effort of 66,391 pounds. Ralph Graves snapped this photo of No. 4307, acquired via eBay, at Kansas City, Missouri on June 27, 1947. All members of the class were dropped from the Frisco roster in 1952.