The I-551 stamp indicates temporary legal permanent resident status, notes Norrie and Associates. The stamp is used in between the time a person receives legal status and the time he receives his alien registration card.
According to Norrie and Associates, I-551 stamps are temporary green cards that provide someone with permanent resident alien status. If a person travels abroad, he is able to reenter the country using the I-551 stamp if he has not received the permanent I-551 card yet. Since the I-551 stamp is temporary, it does expire. It is valid for about a year, but people with an I-551 stamp should examine the stamp for the official expiration date.