The Adventure of the Bridegroom's Photograph (джен)

There is more than one secret to be revealed when a young man comes to Holmes and Watson with a keepsake and a question.
Written for the Spring 2015 round of acd_holmesfest. inamac asked for a historical mystery with an emphasis on detection. See end for notes on the real mystery, what I changed, and how it differs from Arthur Conan Doyle's solution – and he had one!