This determination on SBRs by ATF is unenforceable, arbitrary, and capricious. Please help stop this tyrannical agency by leaving a comment opposing this new rule.
Proposed rules on pistol braces (https://www.federalregister.gov/documents/2021/06/10/2021-12176/factoring-criteria-for-firearms-with-attached-stabilizing-braces#open-comment)
Note that I know of at least one person who uses just this kind of stabilizing brace due to a disability (cerebral palsy).