Apparatus for introducing magnetic spirals into rows of neighboring perforations in the spines of stacks of overlapping sheets which are to form discrete calendars, memo pads or analogous commodities employs an elongated guide for a continuous magnetic coil spring. The coil spring is rotated about its longitudinal axis and is moved lengthwise so that its convolutions enter successive perforations of the row of perforations in the stack being held in the guide. The guide embodies or cooperates with magnets which maintain the convolutions of the coil spring in the path defined therefor by the guide. The magnets can be embedded in antimagnetic tubes forming part of or constituting the guide.