Not a terribly difficult legal question, and almost more appropriately posted in the NFA firearms section, but here seems fine since the appropriate legal answer would be to write the ATF and find out.
Not much discussion that way, but they are very helpful and polite.
If the firearm is a pistol, one may put a
brace on it, as shown in the advertisement, even with a stock adapter. But, one may not put a
stock on it without first filing and obtaining a tax stamp for it.
As long as the copies aren’t sold, copy away.
Imitation is the most sincere form of flattery!
The piece linked to looks as though is needs another piece to attach a ubiquitous pistol buffer tube to.
As long as it is not a rifle stock, a human could use Duck Brand duct tape to attach it with, if they were so inclined.
An interesting question I would have is, if one made their own wire brace at home for it, if our oppressors would allow it to be deemed a brace, but I don’t want to derail so I won’t ask.
