Most listings in the Scott catalog round the perforation rates to the nearest 0.5 None of these stamps are exactly 11 or 10.5, but only close.
For your stamp, note the details at the link here for Perf 11-70 and Perf 10 1/2-75 on the Kiusalas U.S. Specialty Gauge.
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https://www.usca.nl/wp-content/uplo...SA-Gauge.pdfTo really show the rounding done by Scott, note there are three different actual rates which Scott reports as "perf 10" (10-79, 10-80, and 10-81).
Also, make sure your perforation gauge is accurate. Have you measured it against other standards? Some are not truly accurate when made and some plastics tend to shrink over time.
Often the best gauge is to use another stamp of
known rate.For example, Scott 803, the 1/2 cent Benjamin Franklin issue, is also listed as 11x10.5 and will match your Scott 632.