1. Please use tongs rather than fingers. 2. You do not have a perforation error. The perforation rates in catalogs are rounded off to the nearest half. Your stamp is listed as "11 x 10.5", and will be very slightly off from this, but is the closest. The "error" is the rounding by the catalog makers.
Using a modern Scott multi-gauge, I come up with 11.25x10.5 for this issue. Scott has been notorious for rounding perf measurements when doing so does not affect stamp identification. This would seem to be one of those occasions.
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