The "potato" code, yes.
But knowing that Harry is "be prepared" so much that he prepared a recognition code when he didn't think he would ever use it, and then had been nearly a year with a time-turner, and knowning about Oblivatiate, it's quite surprising that he didn't device a recognition code to recognize a message sent either by a future self to his past self (time-turner) or from his past self to his future self (Obliviate).
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 116.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)