Question regarding i551 stamp

[SantaAnaAOS]

So I’ve received my 18 months extension letter and plan to travel abroad. Green card expires on June 25th.
Previously at the border the officer told me that I would need a i551 stamp in my passport once I receive an extension letter. I called today to USCIS to schedule an infopass appointment but customer service gal assured me that I don’t need any stamp and she has no idea why border officer would suggest that.
So she basically didnt let me schedule an appointment.

Did anyone travel just with extension letter? What’s the deal with the stamp?

[newacct]

The whole point of the extension letter is so that it, plus the expired green card, serves as a valid green card (I-551). So you would not need an I-551 stamp, which serves the same purpose. If you had an I-551 stamp, you would not need an extension letter.