in fig9 from which protrude alignment and load transfer tabs 903 and 905 in a first direction and from which further protrude channel base portions 906 and 908 at edges 915 and 917 respectively in a second direction lip portions 907 and 909 are attached to channel base portions 906 and 908 to form first channel 912 and second channel 914 respectively while the various parts of side mull coupling member 900 are described as separate entities it will be apparent to one skilled in the art that coupling member 900 can be produced as a single part by for example stamping and bending of sheet metal the formation of alignment and load transfer tabs 903 and 905 can be aided by first forming aperture 923 shown in fig1 and then bending suitably punched tabs to form alignment and load transfer tabs 903 and 905 referring again to fig1 a side elevational view of first mull coupling member 900 shows a typical length to height aspect ratio of first mull coupling member 900 as well as screw holes 1023 for attachment to component unit frames second side mull coupling member 1100 portrayed in fig1 12 comprises base plate portion 1102 from which protrude alignment and load transfer tabs 1103 and 1105 in a first direction and from which protrude spacer portions 1107 and 1109 in a second direction referring to fig1 guide tabs 1110 1112 1114 and 1116 are attached to spacer portions 1107 and 1109 to act as insertion guides during assembly of compound units referring again to fig1 a side elevational view of second mull coupling member 1100 shows a typical length to height aspect ratio.