They used that stamp for decades. It can't be earlier than 1963 but it could be much later into the 1980's, you will need to judge the age of the paper. Glossy on front and paper stock on back (not glossy) means likely 1970's or earlier. Glossy on both sides means 1970's or 1980's. You can narrow it down based on thickness of paper etc., but that is a good general rule of thumb.
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