221B

A small story from the life of Sherlock Holmes at a time when Dr. Watson retired from the problems.

Inspiration... 
http://phantoms.su/topic/54040-kvartira-mistera-sherloka-kholmsa-i-doktora-dzhon...
https://ideas.lego.com/projects/c9047271-83c4-44a9-9384-5b22528a2f4a

Could not figure it out... https://www.twitch.tv/videos/233935882

Hint... Look at the letter carefully.