A machine wherein stuffers are inserted between the panels of folded sheets, while the sheets are confined in receptacles advancing along an endless path, has a rotor which defines a portion of the path and supports several sets of sheet opening elements including swords which penetrate between the panels of folded sheets in adjacent receptacles to open up the sheets preparatory to admission of stuffers between the panels. The swords are mounted on a support which is adjustably secured to the rotor so that all of the swords are adjusted relative to the adjacent receptacles in response to proper adjustment of a single sword. This simplifies the change of setup when the machine is to be converted from treatment of folded sheets having relatively thick panels to treatment of folded sheets with thinner panels or vice versa.