A French company selling a hair tonic under the name Royal Windsor? And it seems this name was used well before the King of England decided to change the family name to Windsor from Saxe-Coburg and Gotha for obvious reasons in 1917. Joseph Jackson of Paris, France, registered this brand name as a Trade Mark in 1892 (TM #21,987). He claimed to have been selling the brand since 1879.
Just as odd are the use of children and steam trains in these promotional cards - wouldn't children have little need for hair regeneration, and how would steam locomotives affect it?