According to "Regulations on Types and Classification," established by Tetsudo-In (the predecessor of the Ministry of Railway) in 1909 after its purchase of Nihon Tetsudo, four locomotives manufactured by Stephenson were classified as Type 120 (No. 120 - No. 123).
この買収を受けて1909年（明治42年）に制定された鉄道院の形式称号規程では、スティーブンソン製の4両は120形（120～123）に改められている。