points a b c and d two of the points namely a and b are present on the leading edge while the remaining points c and d on the trailing edge the points a and c are nearer to the top of the mail item 10 while the points b and d nearer to the bottom it should be noted that the window area 11 has a high reflection factor for regularly reflected light as compared with the remaining area and therefore that the window area 11 reflects light incident thereon both irregularly and approximately regularly it is also to be noted that the light may not necessarily be visible light referring to fig2 a device disclosed in the above referenced patent specification determines the point a as a predetermined point on the leading edge for use as an origin of a coordinate system merely for simplicity of description it is surmised that the coordinate system is a rectangular x y coordinate system having an x axis in the sense opposite to the sense q of transport of the mail items and a y axis perpendicularly of the x axis towards the bottom information positions of the postal information pieces 12 through 16 are equivalently defined by segment patterns l j j 1 2 and 12 the patterns l 1 through l 4 are for the addressee 39 s name and address 12 located inside the window area 11 while the patterns l 5 through l 8 for the sender 39 s.