The true identity of the loco in this picture is that of 6152 'The King's Dragoon Guardsman'. In 1933 no 6152 swapped identities (including names) with no 6100 and was sent to the US on a successful visit to the Chicago World Fair. The loco later went on an 11,194 mile tour of North America which included a crossing of the 'Rockies'. The locos were never reunited with their proper identities and spent the rest of their service as their 'alter ego'. No 46100 carried a bell commemorating its trip.
Since the '6100' in this photo is pictured with this bell it can be deduced that it was pictured some time after its return from North America and is in fact 6152. This loco is now preserved as 6100.
The true 6100, real identity 6152 was,