Sentence:  Prior to the issuance of the first stamps, letters accepted by postmasters for dispatch were marked "Paid" by means of pen and ink or hand stamps of various designs. [...] To facilitate the handling of mail matter, some postmasters provided special stamps or <WSD>devices</WSD> for use on letters as evidence of the prepayment of postage. 
Meaning of the word marked with <WSD> tags:  An image or logo denoting official or proprietary authority or provenience.