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Explanation: After you make a deposit, the teller stamps the deposit booklet to show the time and date, etc. There could also be some confidential information, hence 密.
In American English, stamp (noun) is more normal than "seal" -- seal has too many other meaings. Also, "to stamp" a booklet makes sense, but "to seal" a booklet (i.e., to close it) would be wrong here.