As what he (and Sir Doyle) thought were his last words, Sherlock Holmes wrote, "My Dear Watson, believe me to be yours".
And that, my dear fellows, is a canon confession.
~ Sir Arthur Conan Doyle - The Final Problem (1893)
Yes, "sincerely yours" is a common signing phrase. However, the "believe me" part and the structure of the last sentence are quite interesting. Look how he mentions Mrs. Watson and in the same sentence goes on to say "believe me to be very sincerely yours". That sounds like "I know you have a wife, but believe me, I'm yours".

