Dead wax markings are the manufacturers markings in the dead space between the end of the song and the label. Their appearance can be used to authenticate an item, verifiy the pressing edition and/or identify a specific release.
It depends on the label, manufacturer etc.. some are etched, some are stamped.. just because a pressing uses different deadwax markings doesn't mean one or the other is counterfeit or a repro, it can vary substantially.
Some vinyl on rare occassions bears no dead wax markings, and is still authentic.
A collector generally bases their real/bootleg assumption on a combination of label printing/condition, vinyl weight (something you can only do in person), vinyl condition, and seller reputation.
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