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).