A book that is important, suggest you obtain "Perforation Measurements of United States Stamps" a series of article from the United States Specialist, Bureau issues association!
#1 Specialized perforation gauges.
#2 Determining perforation without a gage.
#3 Perforation measurement and scott 544.
Now then #3 is a detail of my unique discovery of a U.S.A. 544. I bought this stamp for less than 1 cent.
 
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