latch assembly 69 is swung to its rear position and a module 30 is placed into the nest with the lower retainer portions of assemblies 45 resting on supply and return line ports 62 63 referring to fig9 it can be seen that the upper surfaces 78 of the ports 62 63 are spherical in shape and the lower portions 79 of the retainer 51 is conically shaped to precisely position the module on the ports this interface provides constraint in both directions of horizontal translation and in two directions of rotation the module 30 has an opening 100 in the rear wall of its frame 37 and an upper index surface 101 of that opening is constructed to be placed on hooks 77 formed on bracket 65 of nest 60 the lower frame 37 also has an abutment surface 103 forward of opening 100 and the surfaces 101 and 103 of a module 30 cooperate with surfaces 102 and 104 to constrain the print head module in the other pitch rotation direction with the module 30 resting on surfaces 102 of hooks 77 and surfaces 78 of ports 62 63 the latch assembly 68 is moved to its forward position during this movement cam surfaces on the lower portion of arm 68a force the module downwardly to a fully seated position when arm 68a has moved to its over center latch position surfaces 103 104 interface through opening 57 in cooperation with interface 78 79 to constrain the print head vertically as the latch bar moves the module to.