The 2120 class is built by North British in 1905. The 2120 class is the thing that improved 2100 class. His old name was called B6 class, and after renamed 2120 class. The BK80 class of Taiwan is the same class locomotive. A lot of locomotive became needed at the time of the Japan and Russia war. However, it lacked the ability that manufactures this locomotive with only England. Therefore, it was ordered in every country by using the same blueprint. And, the locomotive type of same was built also in other British company(Sharp Stewart, Dubs), America(Baldwin 2500 class, 168 cars), Germany(Schwarzkoff(Berliner Maschinenbau),Henschel,Hanomag, 2400 class, 75 cars), Japan(Kobe works).
This is a very interesting case. Fundamentally it is same the locomotives of any countries.
However, there is a little difference.