machine onto which it is applied in the length 2 the inner projections 2a form the sliding guide for the sensor detecting the yarn reserve while the cavity 2b is adapted to house along the length 1 and of the motor contained therein the cards of the electronic circuits controlling the weft feeder the extruded lengths 1 and 2 are obtained by simply cutting them into the size exactly required for the purposes having to be fulfilled from considerably long extruded sections easily obtainable on the market the length 1 forming the central body is therefore considerably shorter than the length 2 which forms the arm it should be noted that this arrangement is possible thanks to the fact that the cross section of the casing has been divided as shown in fig1 along the zones c and d on the other hand along such zones it is necessary to provide for the tight connection between the two lengths in order to give to the casing of the invention the required monolithic structure this may be obtained in any known manner for instance by simply fixing together the extruded lengths suitably shaped for the purpose as shown in fig5 where the engagement is favoured by the elasticity of the section length 2 or in fig6 where a conventional dovetail engagement is obtained or in fig7 and 8 where a free sliding engagement is obtained between the two lengths 1 and 2 fixing them together by.