All Reference 2403 chronographs use the Landeron 13 movement, but three different versions of this movement were used. The first version of the Landeron 13 movement has a single pusher; a second (bottom) pusher was added in the second version (see Abercrombie sample), with a relatively large space between the crown and this bottom pusher; the third version of the movement (see Aerotel sample)continued to have two pushers, but the space between the crown and the bottom pusher is smaller.