Sentence:  Add five words for address if replies are to come to a <WSD>box</WSD> number address at any of our offices. These replies are forwarded each day as received, in new envelopes at no extra charge. ... When replying to blind ads be careful to put on your envelope the correct <WSD>box</WSD> number and do not enclose original letters of recommendation—send copies. 
Meaning of the word marked with <WSD> tags:  A numbered receptacle at a newspaper office for anonymous replies to advertisements.