Genuine Mi 18 Oddity
The accepted wisdom from the back row of the East Bay Collectors Club is that the single-line perforator got jammed or malfunctioned in some other way, and the operator had to reverse the machine to get the sheets out. In the process, the sheet was rotated and the "reversing perfs" were made at an angle.
Herb Volin has a more precise explanation: the stamp is a foldover freak. It's creased along a line that bisects the angle between the two perforation lines, and the paper is folded over, so that both lines of perfs were made on one stroke.
Sure enough, under a strong magnifier, it looks like there's a crease where Herb predicted.
The performating machine may have jammed at the spot where the perfs stop; or the operator may have noticed the fold-over and stopped the machine.
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This is a left margin stamp which might be platable because of the thick Z and the balls at the top of the "2's".
See other oddities: Cat #'s 2, 3, 4, 5, 6, 7, 8, 15, 16, 18.