must have had something enclosed to end up in the minor dead letter office
Of course the letter remains sealed until it gets to the DLO, which opens it and determines whether names or addresses are inside, the type and value of the contents, etc., and sends the letter to the correct division for action.
Marshall Cushing's valuable and informative book "The Story of Our Post Office" published in 1893 has a chapter "Dead letters and live ones" running between pages 242 and 276 describing the DLO operations (in Washington, DC, I think). A brief excerpt from pages 263-4 mentions and describes the "Minor Division", as opposed to the "Money Division":
The "B" is added as a separate handstamp within the DLO octagon, matching the "Batchelor & Co" address, for filing and finding purposes.
Quite collectible, not too valuable.