His last could have either been the letter he left with Harry when Harry was dropped off at their house 15 years prior, or when Dumbledore replied to Petunia years ago, when she was trying to get into Hogwarts, that she could not do magic, but be use in the muggle world.
Personally, I think it was the letter that Dumbledore left with Harry when he was dropped off 15 years prior.
Yeah. I think that's it.
I think I expected more to come of it. But maybe it's implied in the reminder that the protection vanishes when Harry turns 17.