two covers dated April 12, 1847 and May 24, 1847, first with "5" rate handstamp, second with straightline "Paid" and "10", very fine and scarce, Merrimack's post office was established August 19, 1846 and its name changed to Lawrence just nine months later on May 26, 1847, the May 24 cover is the latest recorded use of this datestamp