This plaque commemorates the bicentenary of the birth of Anthony Trollope (1815 - 1882), who introduced pillar boxes to the United Kingdom in 1852.
In I855 this street became one of the first in London to have a pillar box.
5 similar plaques have been erected.
Site: Anthony Trollope - pillar box - Pall Mall (1 memorial)
SW1, Pall Mall
Nothing to do with this memorial but interesting nonetheless, while researching this pillar box we found a March 2019 demonstration on Google Street View.