Well since the manufacturer sells it just as a "parts kit," one could complete the "kit" with whatever parts they choose, including a pistol brace in lieu of the wire stock. Basically, I'm not seeing how the configuration the seller is selling the "parts kit" n has any relevance, as the wire stock is just one of many parts contained within their "parts kit."
Just my $0.02
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Agree. IANAL, but putting a stock on a not-a-firearm shouldn't affect it as a firearm.