Abstract: A clamping device which holds one half of a folded jacket against an inclined platen as an insert is dropped into the jacket against the fold. The platen is one of a large number of similar elements carried by a carousel which intermittently shifts the platens from station to station, the clamping device being rendered operative at one station and continuing to remain operative through several stations as one or more inserts are dropped successively into the jacket. The clamping device prevents the pages of the jacket from blowing open as inserts are dropped. The clamping device constitutes a flexible resilient arm, one end of which is fixed to an oscillating clamping bar that reciprocates the clamping device between an idle and an operative position. The tip of the arm supports a resilient pad that, when the arm is in operative position, presses against the inner surface of the jacket half that rests against the inclined platen.