求翻译：买方须于2003年4 月30日前通过开证银行开出以卖方为受益人的不可撤消60天期信用证，并注明在上述装运日期后21天内在中国议付有效，信用证须注明合同编号是什么意思？

The Purchaser shall on April 30, 2003 by the issuing bank to open an irrevocable 60 days of the letter of credit in favor of the seller, and indicate within 21 days after the above date of shipment for negotiation in China, the letter of credit shall be marked Contract Number

In 2003, the purchaser shall be 4 30 months passed before the issuing bank on out to the seller for the benefit of the 60 days cannot be reversed, and the letters in the above shipping within 21 days after the date the payment in China, credit should be noted that contract No.

The buyer had through to open the card bank before April 30, 2003 to start out take the seller cannot abolish 60 day time letters of credit as the beneficiary, and indicated in the above shipping date latter 21 day intrinsic China negotiation is effective, the letter of credit had to indicate the co

Buyers are required to depart by the issuing bank to the seller before April 30, 2003 for the beneficiary of the letter of credit cannot be revoked 60 days, and indicate in the said Chinese negotiation within 21 days after the date of shipment is valid, should specify the credit contract number

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