The device comprises a box-like body inside which two shoes are slideable and are actuated in opposition. A shoe is compsed of a plate which has a guiding opening for a gib of the other shoe. Brackets are rigidly and protrudingly associated with the top of the gib and with the lower end of the plate and bear respective jaws which are adapted, in jaw closure position, to define a guiding duct for a bar and, in opening position, a passage for the bar-pusher. The opposite actuation of the shoes is actuated by a shaft controlled by a fluidodynamic jack having arms which bear wheels acting between the bracket supported by the gib and a protrusion of the plate.
