A stacker for sheets, e.g. of cardboard or paper, is equipped with a device for inserting Marker strips or tabs within the stack to indicate predetermined numbers of sheets therein, e.g. for identification of reams. The inserter has a rigid channel Mounted on a frame moveable on a traverse transverse to the direction of sheet feed and the channel at its outlet is rigidly connected to a tongue having a substantially horizontal underside and moved with the outlet of the channel in a rectangular pattern with only vertical and horizontal path segments so that the tongue can be withdrawn from the stack without changing its orientation horizontally after a strip has been inserted into the gap formed by the tongue in the stack and alongside the tongue.