upper portion 20 and the upper surface of lower portion 24 in the preferred embodiment depicted in fig1 background member 14 is configured to complement the structure of display member 12 so that display device 10 has a generally rectangular overall vertical cross section this is not the only possible embodiment however in other embodiments not separately shown background member 14 may not completely encapsulate the sides of display member 12 but rather may be co terminous therewith furthermore the top of display member 12 and background member 14 need not be co planar the possible alternative embodiments are also discussed in our earlier patent and so are not discussed further here the primary difference between the device disclosed in our earlier patent and the instant device relates to the configuration of first locking element 18 hereof and so the following discussion will be directed to that element while background member 14 includes a second locking element 26 which differs from the analogous member of our patented device the shape thereof is determined by the shape of first locking element 18 so no separate discussion thereof is believed necessary turning now to fig2 first locking element 18 is shown in further detail fig2 is a bottom plan view of display member 12 with indicia 16 omitted for clarity while indicia 16 and the remainder of display member 12 are described as though they are discrete components it will be appreciated that in fact.