Many people on internet hide s/n because they saw other people doing that. This illness propagates so quickly in the internet era, so even newly acquired brand-new shotgun posted having s/n masked on pictures, for God-knows-what-is-that "security" reason
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[Edit] In collecting context, could this be possible, say, a 70% gun with s/n exposed one day. Months or years later, another 95% one appears on market with same s/n ?? This must be relatively more common than a gun being on a stolen list in its history.