alub, The story behind the "three rows" stamps is what makes your stamp so interesting.
After the stamp designs were transferred from the transfer roller to plate 3, the spacing between the eighth and ninth vertical rows of the left pane got gradually too wide toward the bottom, and the spacing between the ninth and tenth vertical rows was too narrow. To make the alignment errors less conspicuous when the sheets were printed, extra frame lines were cut into the plate between the eighth and ninth vertical rows at most of the horizontal rows. Conversely, the side frame lines were cut abnormally close to the design on some positions between the ninth and tenth vertical rows, and the inner frame line was omitted for some positions.
Here is a diagram from Carroll Chase's book that shows the unusual frame lines of the three rows: