Also, Lily called Sirius 'Padfoot' in the letter, the name only his closest, dearest friends knew. The Marauders were such a tightly knit family, Sirius would certainly have mourned deeply enough to carry the letter even through Azkaban. Gonna go sob now.
Sirius-Harry: "You're not a bad person. You're a very good person, who bad things have happened to. Besides, the world isn't split into good people and Death Eaters. We've all got both light and dark inside us. What matters is the part we choose to act on. That's who we really are." ♥