Danvers, MA, postmark handstamp
The Post Office Department's 1895 contract with Benjamin Chambers, Jr., for postmarking handstamps describes this marking device as item 174: "'Stamp J' consists of an oval wrought iron die, the oval to be 1- 3/16 inches at its widest point and 13/16 inch at its narrowest. The name of the office appears in relief, running parallel with the sides of the oval die."
Mail other than first-class was postmarked with the town name and state but not the date. This marking identified the place that mail entered the mail stream.