27 the contiguous cut outs or openings 76 and 77 open into each other along one longitudinal side and have their end edges offset from each other more specifically the front and rear end edges 78 and 79 are respectively adjacent to and offset rearwardly from the front and rear end edges 80 and 81 of the opening 77 the contiguous rectangular openings 76 and 77 are displaceable upon shifting movement of the plate 66 between the positions of fig5 and 3 to respectively overly the space between the framework walls 26 and 27 interposed between the framework walls 26 and 27 is a radial member or crank 83 nonrotatably circumposed about the tilt rod or shaft 20 and having an arm 84 projecting radially upwardly through the cut out 75 of operating member 66 the radial arm 84 projects into one of the rectangular openings 76 and 77 depending upon the position to which operating member 66 is shifted for example with the operating member 66 shifted rightward as seen from the front or right hand side in fig3 the arm 84 projects upwardly through rectangular opening 77 in this condition the tilt rod is rotatable between limiting positions determined by abutting engagement of the arm 84 with end edge 81 as shown in solid lines and end edge 80 as shown in phantom this may be considered the winter position of adjustment of the tilt rod limiting structure 25 for example the several slats 13 see fig4 may have their upper surfaces 86 reflective as by aluminum or other reflective coating and may have their lower surfaces 87 absorptive as by a dark colored coating in the solid line limiting.