Kit of parts comprising an elongated track and a plug: the elongated track extending along a length axis in an axial direction, having a first and a second end and comprising an elongated slot formed by a central groove with at least one shielded lateral groove (11,13), said slot having a slot cross section and an open side opposite a bottom surface, said central groove and lateral groove extend next to each other in a plane P, at least two electric conductors (15,17) are accommodated in said at least one lateral groove, said slot, open side, central groove, at least one lateral groove and electric conductors extend in axial direction essentially from the first end to the second end, a connector portion of the plug extending along a connector axis, having a first and a second, oppositely arranged main surface, and having a connector cross section which matches with the slot cross section, wherein both said slot cross section and connector cross section are asymmetrical with respect to a plane Q extending over the length axis perpendicular to plane P, and at least the slot cross section is mirror asymmetrical with respect to said plane P, and the slot comprises at least one off-center located profiled structure extending in axial direction over essentially the whole length over the bottom surface of the slot, and each main surface has a corresponding inversed profiled structure provided on the first and second main surface in mutually mirrored position with respect to plane P, characterized in that the connector cross section is symmetrical with respect to plane P for features of the connector cross section which are critical for a mutual match with the inverse features of the track for rendering the connector portion insertable in both the first and second end of the elongated track with the correct polarity by being rotated over 180° over a normal axis perpendicular to plane P. the elongated track extending along a length axis in an axial direction, having a first and a second end and comprising an elongated slot formed by a central groove with at least one shielded lateral groove (11,13), said slot having a slot cross section and an open side opposite a bottom surface, said central groove and lateral groove extend next to each other in a plane P, at least two electric conductors (15,17) are accommodated in said at least one lateral groove, said slot, open side, central groove, at least one lateral groove and electric conductors extend in axial direction essentially from the first end to the second end, a connector portion of the plug extending along a connector axis, having a first and a second, oppositely arranged main surface, and having a connector cross section which matches with the slot cross section, wherein both said slot cross section and connector cross section are asymmetrical with respect to a plane Q extending over the length axis perpendicular to plane P, and at least the slot cross section is mirror asymmetrical with respect to said plane P, and the slot comprises at least one off-center located profiled structure extending in axial direction over essentially the whole length over the bottom surface of the slot, and each main surface has a corresponding inversed profiled structure provided on the first and second main surface in mutually mirrored position with respect to plane P, characterized in that the connector cross section is symmetrical with respect to plane P for features of the connector cross section which are critical for a mutual match with the inverse features of the track for rendering the connector portion insertable in both the first and second end of the elongated track with the correct polarity by being rotated over 180° over a normal axis perpendicular to plane P.