There’s a dissatisfying circular argument wrapped up in the “in common use” test for legality: items which are restricted will be largely avoided by most of the general public, e.g. because they don’t want to do the NFA paperwork. This avoidance makes them less “in common use” - which the authorities then seize upon, turn around, and claim that the fact they are not in common use therefore legitimizes and even justifies the ban! But that’s a circular argument, because the lack of common use arose, at least in part, because of the ban.
Circular arguments are terrible foundations upon which to build legislation.
This is a rare case where that cycle is broken: given the block (NFA) preventing common use, a loophole was found and exploited by the public to satiate the common demand. We can now subpoena the sales figures for an identifiable specialized product (braces), fairly estimate that only a tiny fraction of those sold were for the “intended purpose” (handicap use), and reasonably conclude that upwards of ~1% of total population acquired it for a singular purpose: constructing what is, for all practical purposes, a short barreled rifle - in quantities that must be considered “common use”.