Is it a common practice for a manufacturer refurbished USA Jackson guitar that has a "used" stamp on the back of the headstock to have the serial number removed/erased from the guitar? I was told that this is to ensure that the warranty was voided, but it sounds a little suspicious. Thanks to anyone with an answer.

Yes, that is the standard practice for all major manufacturers. If you look closely at the end of the fingerboard, you can probably see where the serial # was removed.

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