The device includes a planar body (3) secured to a harness bar (2), a hook (10) pivotably mounted on the front portion of the body (3) about a pin (9) transversal to the body, wherein said hook comprises a pointed end (13) on the rear side thereof, a hairpin spring (15) with the facing ends of the branches thereof engaged in apertures (16) of the body that are offset so as to rotate the entire spring unit forwards, and such that the central portion of the spring (15) locks the hook (10) by engaging same at the rear of the pointed end (13) thereof, and a lever (11), hinged to the body (3) about a pin (17) parallel to the pin (9) of the hook (10) and located above same, comprising an upper portion (18), above the pin (17), facing upwards and forwards and a lower portion (19) facing downwards and rearwards, the lower end of which engages the forward-facing surface of the branches of the hairpin spring (15).