Little to add to Tom's information but a bit more, and some Hunter Arms' ads here
http://docs.google.com/document/pub?id=17ixogftgITEblNUWtmFBv96ZvgjK6eFell8GsAWd-KICrown steel first appeared with the Pigeon Grade in 1893, No. A 1 (SN 1130) in 1894, was also used for the No. 3 about 1895, and in 1898 for the No. 2. Nitro steel was introduced in 1898 for the No. 3 and Pigeon and later the No. 4 and No. 5. Armor steel appeared in 1898 with the 00, and Royal steel for the F grade hammer gun in 1901. London steel was used for the No. 0 and Special steel for the No. 1 in 1907.