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Well.. the envelope is issued, so logic says check with the embassy and not based of what you think and what you read. That envelope has instruction in the front "Do not open, or tamper."  Makes sense?

The triangular tear about 1.5 or 2 inches at the top right or left corner of the envelope is normal. If you see the tear to be longitudinal, don't take risk of traveling with it. Report it to your embassy immediately and have them replace the envelope. The airline flight attendants are so ####### about this envelop. They have the right to offload you or deny you from boarding your flight to US. 

"You entered on an IR1 and you are a legal permanent resident on day 1 of your entry to the US. Your green card ID can expire but your legal permanent status is still in effect. If you have submitted your I-751 and did not retract it, then you are in good hands." This advice came from the immigration lawyer I consulted this year.