Devices for fixing objects to metal plates accessible from only one side

A device for fixing an object by screwing it to at least one blind plate or similar support having a mounting slit extending therethrough with parallel edges of width a, comprising two independent screwing elements adapted to cooperate with one another, the first of which comprises a cage made from a plastic material formed by a cylindrical socket extended by a radial collar, said socket having, on the one hand, at all its points in a transverse direction a dimension at most equal to a and, on the other hand, axailly in the immediate vicinity of said collar a groove which extends along a geometric closed line inscribed in a square whose side is equal to a, this line comprising two opposite corners of said square and the two half-sides extending from each of these two corners, wherein the first screwing element comprises further a threaded metal part housed in the cage and wherein the closed line comprises further two opposite arcs of a circle of radius a/2, centred at the centre of the square, extending respectively from the two opposite ends of said two half-sides which are perpendicular to said transverse direction, the end of each arc of a circle the furthest removed from the middle of the side of the square where this arc originates being connected to an eccentric boss.