Dogtown and 9x19, thanks for pinch-hitting for me during my brief absence.
"There is no definitive answer" is the short answer.
The longer answer is that the ATF has only given approval to specific types of Stock-Like Pistol Thingy (which I'll call SLPT for short
). As far as I know, they have NOT issued any dimensional guidance regarding where the magic line is drawn between an SLPT and a shoulder stock requiring a tax stamp.
If you choose to roll your own SLPT without prior ATF approval—even if it's a close copy of an approved type—you're in uncharted territory and you must be prepared to suffer the consequences (+1 Dogtown).