The No. 235 & No. 335 doubles were introduced about 1908 with Compressed Forged High Pressure Steel.
Western Field, November 1908
https://books.google.com/books?id=UB0LAAAAIAAJ&pg=PA197&lpg

1917 Stevens "Riverside" No. 225 hammergun with "Nitro Steel"



In the 1926 Hibbard, Spencer, Bartlett catalog the No. 315 Riverside hammerless is listed with Blue Steel while the No. 330 Stevens hammerless with High Pressure Compressed Steel, tested with nitro powder.