Stamps (Philately)/Possible Defect

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Question
Some of the "MNH" stamps that I have bought have two parallel slightly raised lines on the reverse side.  Is this a hinge mark or perhaps some other defect in the gum?

Thank you.

Answer
Tom,

It reads like you are describing gum breakers.  Gum breakers were purposely introduced into the rotary press stamp printing process starting in the 1920's to prevent the sheets from curling.  The technique had limited success and was eventually phased out.

-Mark Leon

I deal in United States Stamps
http://www.forwardedge.com/usmints.html
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Stamps (Philately)

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Mark Leon

Expertise

I am a collector of United States postage stamps, and also a stamp dealer. I`m a member of the American Philatelic Society (APS). My principle area of interest and expertise is in United States Stamps.

Experience

I've been selling stamps on the internet since early 1996. I maintain a stamp web page that concentrates on mint US stamps issued generally between 1890 and 2009. I'm an active seller on ebay.com since 1996, under the user id "mleon". You can find my web page at http://www.forwardedge.com/usmints.html

Education/Credentials
I have a Bachelor of Science degree in computer science from the University of Nevada at Las Vegas, 1984.

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