Apologies, the word I missed out was "why" "it was classified as a rebuild" (did the old capital accounting shenanigans still count? Did Thompson want to avoid accusations of building lots of new classes? It's got a B1 boiler, B1 cylinders/valvegear etc(?), presumably the new frames were B1 pattern. I would have thought it was better classfied as a B1 (or even a sub-variant of B2) with additional bits.