Device for attachment to a fastening rail

Attachment device comprising a base member (1, 21), a cap member (9, 19) and an attachment member (12, 32), said base member comprises a non-circular head (15, 35), a threaded opening (3, 23) extending axially through said head, and at least one first anti-rotation surface (24), said cap member (9, 29) comprises at least one axially extending through opening (13, 33) and at least one second antirotation surface (10, 30) fornon-rotational engagement with another surface, so that said first and second antirotational surfaces rotate as a unit, said cap member posirioned on said base marker wherein the improvement comprises that said attachment member (12, 39) has a non-circular opening (14, 37) for affording passage therethrough of said head (15, 35) of said base member (1, 21) in a specific angular position, a flexural member (5, 27) extending generally in the axial direction of said threaded opening (3, 23) and positioned adjoining at least one of said base member and cap member, said flexural member being spring deflectable transversely of the axis of said threaded opening (3, 23) in said base member (1, 21), and said flexural member has a radial outside surface in an unloaded condition projecting at least partly outwardly beyond an axial projection of said non-circular opening (14, 37) of said attachment (12, 39).