'Sherlock' photo shows Holmes and Watson in Victorian attire

The BBC has posted online an intriguing photo of Sherlock co-stars Benedict Cumberbatch and Martin Freeman, but offered little explanation about what it means.
The pair play sleuths Sherlock Holmes and John Watson on the popular TV show, which is set in contemporary London and is based on Sir Arthur Conan Doyle's late 19th- and early 20th-century mysteries.
Sherlock is scheduled to return for a 2015 Christmas special and a fourth season in early 2016.
The photo BBC One posted this week on Twitter shows Cumberbatch and Freeman dressed in Victorian-era clothing. The mysterious tweet also features the hashtags "221back" and "not kidding."
221B Baker Street is Holmes' famous address.
#221back #Sherlock #notkidding pic.twitter.com/iTfDqfJI0R— BBC One (@BBCOne) November 25, 2014