I my haste to complete the post I omitted mentioning straight edges. Flat plate straight edge sheet margin stamps may capture the center line if the margin is wide enough. Rotary press sheets used to produce sheet, booklet and coil stamps do not have a center line. Coil joint lines show at the edges of abutting plates while flat plate center lines are engraved into the plate. Since the stamp shows the center line at the edge of the right margin it must be flat plate.
While the top and bottom perforations of rotary press sheet stamps must be aligned, they may be aligned, but are generally not. As far as the appearance of the printing is concerned, in addition to the center line, the color is still a deeper green than most rotary press stamps. The lack of setoff could occur if the sheet was at the bottom of the pile or if drying conditions were better than normal.
Some 1 cent stamps, especially flat plate dry prints, may resemble rotary press stamps closely enough to require closer examination. However, because this stamp is a straight edge with part of the center line visible, it is clearly flat plate.