Notecard about a rendezvous at 15 Buckingham Palace Mansions (ca. 17 december 1925)
From The Arthur Conan Doyle Encyclopedia

This notecard was written by Arthur Conan Doyle from Windlesham, Crowborough, Sussex, a few days before thursday 17 december 1925.
Notecard
I can see you at 15 Buckingham Palace Mansions (opposite Victoria Station) at 10 o'clock on Thursday morning next (Dec 17th).
Arthur Conan Doyle