3GS 4.3.1 on 6.15. Want to restore to factory

[andrew5826]

hey i have a 3gs FW 4.3.1 BB 6.15. i'm not sure if its new bootrom or old bootrom but the serial num is 880270893NS.
everything is working but i want to restore to factory settings so do i need to restore with a custom firmware? is there a way to restore without having cydia on it? does anybody have a custom firmware that will work on my 3gs without installing cydia?
Thanks

[i.Annie]

With that baseband I don't believe there's any way to restore to factory settings. You'll always have to use a custom firmware.