In examining the edges of the paper, I noticed that they were more worn than seemed necessary.
It looked like the paper had been folded once a certain way, and then folded in the reverse direction in the same fold as the original.
It was clear to me that the letter had been turned inside out, a new address had been printed on the reverse side and then it was sealed.
This was enough to make me sure beyond any doubt that it was the stolen letter.