Abstract: In a duplicating machine, a system for readily interchanging a printing head on a permanent rotatable shaft of a printing couple of the machine, to facilitate rapid changing of printed indicia in short run duplicating situations. An elongated manually manipulatable segment is readily removably mountable on the rotatable shaft. At least one ring-like mounting member is fixable to the manually manipulatable segment at any position therealong. At least one adjustable printing head is secured to the mounting member. The manually manipulatable segment can be bodily removed from the shaft and the printing head can be replaced or adjusted while the machine continues to operate.