The Thompson Center case was a bit different. You can assemble a pistol into a rifle, and back - as long as you never assemble an SBR in the process.I believe TC won in court on this, and got it relaxed some.
If the ATF were to consider a brace as a stock - you could own a pistol (no stock and <16" barrel), an upper with a 16" barrel and a brace/stock. Or a pistol with no stock, and a rifle with a stock or; brace and a stock.
Constructive intent would be where you only own parts to assemble an illegal configuration, as in a <16" barrel upper, a compatible lower and a compatible stock (with no 16" compatible upper).