Sentence:  "Dear me, Mr. Holmes. Dear me!" said this singular epistle. There was neither <WSD>superscription</WSD> nor signature. I laughed at the quaint message; but Holmes showed unwonted seriousness. 
Meaning of the word marked with <WSD> tags:  Something written (or engraved) on the surface, outside, or above something else; specifically, an address on a letter, envelope, etc.