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[Karos SkyArmy]

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[LeoRex]

well... towards the end of 2013, Samsung pushed a JB update that locked the bootloader down pretty tight and, I believe, no one ever managed to work around it. There was quite a few discussions about making sure if you picked one up, you told the sales rep to not power it on and let it update (since the 'factory' version was always an old, unlockable build).

If your phone had an update after Nov or Dec 2013, it's might be permalocked... But you might be able to find something to get through it. I don't know where it lead as I traded in my (unlocked) SIII for a Nexus 5 a month later.

 

[LeoRex]

Oh... you MIGHT be OK, depending on how you flashed the stock ROM... if you used ODIN and loaded an official, that may have re-locked it.

There should be a way to check it's state... but being an older phone, there's a lot of stale (mis)information abound. Go do some searches... XDA Developers might be your best bet.