pair of elongated longitudinally extending substantially straight cooperating slots 35 and 36 which are formed adjacent the opposed side edges of each display holding member in addition display holding members 23 30 each incorporate a pair of track engaging fingers or hook members 37 and 38 each of which are positioned directly adjacent one of the elongated slots 35 and 36 as shown in fig4 c the track engaging fingers hook members 37 and 38 may comprise a u201c v u201d shape as opposed to the u201c c u201d shape shown in fig1 4 as shown in the drawings by lockingly engaging finger or hook member 37 of display holding member 23 in slot 35 of display holding member 22 while also lockingly engaging finger hook member 38 of display holding member 23 in slot 36 of display holding member 22 display holding members 22 and 23 are cooperatively associated with each other by employing this construction display holding member 23 is able to advance longitudinally in a substantially straight path relative to display holding member 22 by pulling the leading edge of display holding member 23 causing fingers hook members 37 and 38 to slidingly advance along the length of elongated slots 35 and 36 using a similar construction fingers hook members.