Alford Image Library

I have similar photographs stating "under royale patronage - R Milne, successor to W Watson, portrait and landscape photographer, Ballater". This is in Aberdeenshire, Scotland-Aboyne is 8 miles from Ballater, so is likely to be the same photographer. It would seem that W Watson passed away in 1924 or 1925 (third last sentence on page):

notesonphotographs.org/index.php?title=British_Journal_of_Photography_Almanac_Annual_Summary_of_Photographic_Inventions_and_Events_in_Photographic_History/1925

Unfortunately with no age stated or cause of death, this doesnt make it easy to work out when Milne took over. If he took over upon death, its after 1924 or 25-obviously! :)