South Merrimack, NH duplex handstamp
This is a 'four-bar' rubber postmarker commonly used at third- or fourth-class post offices. It derives this nickname from four parallel killer bars that extend to the right of a circular datestamp. The town name and state appear within a circular border. There are four slots in the postmark's center that hold rubber pica type for month, date, time, and year.
This device style changed very little throughout the twentieth Century. The majority in use were manufactured by the Baumgarten Company, Washington, District of Columbia. The rubber marking die is affixed to a foam rubber cushion with rubber cement. The cushion is attached to a steel plate using the same type of adhesive. Finally, a turned hardwood handle is pressed upon a steel shaft that is attached to the plate.