In any large transport fleet it becomes important to be able to identify individual vehicles in order to maintain records of servicing and any failures experienced. On the railways, everything has a Unique number from the largest locomotive to the smallest track maintenance vehicle. The allocated number usually stays with the vehicle throughout its life unless significant changes are made to its mechanical condition or use, when it might be renumbered to separate it from other un-modified members of the class.
Very occasionally an unusual event results in a renumbering. In 1933 the Royal Scot attended the ‘Century of Progress’ exhibition in Chicago and also toured the USA and Canada. Although not officially recorded, it has become accepted fact that 6152 and 6100 swapped identities to allow the newly built locomotive to attend the exhibition. You can read about it on the Royal Scot Locomotive and General Trust website.