I received the same sort of letter from Synchrony at box 1448 in Southeastern, PA too. The one reason I think it might be legit is, they're not asking for any new information and they aren't asking for money. The reason I'm worried, however, is there is no email address or phone number to call. Ultimately, I'm not terribly worried and I'll send it in because I'm not sharing any (new) info that they didn't include already.
My unclaimed property doesn't appear in the state database yet, because it hasn't been turned over to the state yet. 🙂 I figure, if I don't send this letter back, eventually this $ will show up in the database and I'll be able to claim it then.
I did email the state's unclaimed property email, just to see if they know anything about this letter. I haven't heard back yet.