A few baseband questions

[Guest v1nsai]

I'm getting FOTA errors when trying to downgrade from o2 1.6 to at&t 1.6 . I'm thinking it may be because my baseband isn't what it should be, I've got the 21 and I'm pretty sure the at&t has the 31. Was wondering if someone could verify which baseband each version has, and if I do have the wrong version help me upgrade it. I've been following the instructions in this thread and it's not working.

Help appreciated >.<