Sopotsopot, this stamp was printed in large sheets of 400 which were then cut into panes of 100. The sheets were cut ( supposedly ) on the arrow that you see on your stamp - the person who cut the sheet of your stamp had a hard night obviously
Slight correction for Petert4522 - these stamps were printed in sheets of 200 and cut along the center line into two panes of 100. There were arrows as shown in the scan at the top and a similar one at the bottom. Natural straight edges can be found at the left or at the right. Straight edges at the top or at the bottom are probably unnatural.