the indicators placed upon rotatable cylinders 702 and 704 which are rotatable about axes 703 and 705 respectively the central gear 712 is shown to be rotatable about an axis 713 this rotational movement being coupled to cylinder 702 and 704 via gears 715 and 717 respectively gears 715 and 717 are in turn coupled to smaller gears 719 and 721 respectively which are attached to cylinders 702 and 704 thereby rotating cooperatively therewith slider 730 is provided with teeth 732 which mate with the teeth on gear 712 to distribute the mechanical energy required to turn the indicator bearing cylinders depending upon the overall geometrical requirements of the marker and the length of the slider more or fewer gears may be used to sufficiently turn the cylinders so as to properly align them relative to windows 750 and 751 shown at the rightmost and leftmost ends of the marker housing in fig7 these windows may be large isolated windows or may take the form of elongated slits as shown explicitly in fig1 depending upon the precise indicator implementation in fig7 three different indicator surfaces a b and c are disposed on the walls of cylinders 702 and 704 though any arrangement of two or more such different surfaces or attributes are possible with slider 730 positioned as shown in fig7 surface 34 b 34 is visible through window 750 whereas surface 34 a 34 is viewable through window 751 with precise management of the changing device as it passes over a marker constructed in accordance with fig7 one of the three visual attributes will be visible through both of the window 750 and 751 though with the configuration shown at no time.