According to the invention, the assembly comprises a mechanical connector provided with two cylindrical protuberances and an adaptor (50) with two side walls (54, 55) each having a lateral orifice (54A, 55A), wherein the end of each cylindrical protuberance has at least one portion which is inclined in relation to the insertion direction of said mechanical connector, towards the inside of the adapter (50), and a portion (54B) of each side wall (54) has a cavity (54B), of which at least one inner surface (54C) is inclined in an increasing fashion from the end of said side wall (54) up to said corresponding lateral orifice (54A), and of which at least one lateral delimitation (54D, 54E) is inclined in relation to the plane formed by the axis of said corresponding lateral orifice (54A) and said insertion direction, in a decreasing fashion from the end of said side wall (54) up to said corresponding lateral orifice (54A).