wall surfaces of the collar and socket members illustrated in fig2 for example the cross member 14 can be tilted such that its central axis is inclined relative to the central axis of cross member 12 the frame members 60 62 at the upper end of cross member 14 and the frame members 68 70 at the lower end of cross member 14 are resiliently inwardly compressible to allow a coaxial registration between the midportions of cross member 14 and those of cross member 12 due to the integral molded construction of plastics material the frame members 40 42 and 48 50 at the lower and upper ends of cross member 12 are resiliently outwardly expandable to facilitate the coaxial registration of the midportions of cross members 12 14 if desired only one of the four midportions described above need be made resilient to allow the coaxial registration necessary to form the continuous bushing receiving passageway after inserting cross member 14 within cross member 12 and interlocking the cylindrical walls with the recesses at the upper and lower midportions of the cross members the cross members are rotatably interlocked one with the other so as to be rotatable between the.