The original form of wallpaper, which was made from pulped cotton, was called Lincrusta and launched by Frederick Walton in 1877. Thomas Palmer then developed variations called Anaglypa and Sugalypta in 1887. The two hardwearing relief wallpapers of Cameoid and Lignomur, which we have in number 19, were then launched as competition relief decorations, some designs developed and advertised specifically for wallpaper panels as seen in our house.
The initial knockthrough between the two houses of 19 and 18a happened around 1885-1888 when Bishop Ryle was already in residence ( I'll go through the whole timeline in the book), and this places the existing decorations in exactly the timeframe we might expect. The twentieth century redevelopments of the University altered the size of the doorframe, so the installation of this feature must have occurred between 1887 and around 1905, and would have been either under Bishop Ryle or Chavasse.
Given the emphasis on expenditure at the time of Ryle's retirement in 1900, and the ongoing redecoration of the Bishop's Palace ( including the removal of the name on the outside of the building), the likelihood is that this cheap, hardwearing but elegant wallpaper was installed in 1900/1901 in time for the residence of the incoming Bishop Chavasse.