Union, ME, duplex hammer handstamp
This postmarker was manufactured circa 1900-1931. The nine-bar oval steel die is secured to a cross-bar, with the mortised postmarker steel head mounted in parallel. Earlier styles featured a brass tube instead of the nine-bar oval die. Cut-to-length cork, rubber, or tightly-rolled paper were inserted in the tube and used to obliterate the postage stamp.
Postal employees who postmarked mail found that the task was simpler with a hand movement allowed by a long handle, akin to hammering. This long-handle handstamp was a postal employee's modification. Not until the Pitney Bowes Model S-55 handstamp was introduced were postmarking devices supplied with an extended handle.
The employee modification consisted of removing the usual vertically-placed wooden handle and ferrule. This left an upright spike projecting from the back of the postmarker head or cross-bar. The spike could be threaded or placed through a split at the end of the handle that was bolted together to secure the head in place.
Bond, Arthur H. "Time-saving, Duplex Handstamp; Its Invention, Use and Manufacture." Postal History Journal 10 (June 1963): 59-63.