A method for producing a modular electrical connector assembly kit, comprising: providing a plurality of modular contact sub-assemblies, wherein each contact sub-assembly is a modular component comprising a base component, a pin plate through which contact pins are received, wherein different combinations of number, size, or arrangement of contact pins configured on the pin plate define different contact element configurations (A, B, C) for each contact sub-assembly; selecting from any of the possible configurations (A, B, C) a configuration to define a desired pattern; fitting a first common perimeter-sized contact sub-assembly with any one of the possible configurations (A, B, C) into a first receptacle space of a common insulative material header component; and attaching the first common perimeter-sized contact sub-assembly to the common insulative material header component; fitting a second common perimeter-sized contact sub-assembly with any one of the possible configurations (A, B, C) into a second receptacle space of the common insulative material header component; and attaching the second common perimeter-sized contact sub-assembly to the common insulative material header component; wherein the first and the second common perimeter-sized contact sub-assemblies have different contact element configurations (A and B, B and A, A and C, B and C, or C and B), and wherein fitting a combination of common perimeter-sized contact sub-assemblies into receptacle spaces of the common insulative material header component to define a desired pattern comprises defining a pattern that includes a third but empty receptacle space in the common insulative material header component. providing a plurality of modular contact sub-assemblies, wherein each contact sub-assembly is a modular component comprising a base component, a pin plate through which contact pins are received, wherein different combinations of number, size, or arrangement of contact pins configured on the pin plate define different contact element configurations (A, B, C) for each contact sub-assembly; selecting from any of the possible configurations (A, B, C) a configuration to define a desired pattern; fitting a first common perimeter-sized contact sub-assembly with any one of the possible configurations (A, B, C) into a first receptacle space of a common insulative material header component; and attaching the first common perimeter-sized contact sub-assembly to the common insulative material header component; fitting a second common perimeter-sized contact sub-assembly with any one of the possible configurations (A, B, C) into a second receptacle space of the common insulative material header component; and attaching the second common perimeter-sized contact sub-assembly to the common insulative material header component; wherein the first and the second common perimeter-sized contact sub-assemblies have different contact element configurations (A and B, B and A, A and C, B and C, or C and B), and wherein fitting a combination of common perimeter-sized contact sub-assemblies into receptacle spaces of the common insulative material header component to define a desired pattern comprises defining a pattern that includes a third but empty receptacle space in the common insulative material header component.