What you have is an excuse for the ATF to go rummaging through all your sh*t looking for something to tag you with. :0)Question:
If you have a Glock pistol, and it's not already registered as an SBR,
and you don't own any registered SBRs,
and you order this Shoulder stock,
stating that your only intended use for it is to, one day in the future,
install it on your Glock...
... do you have "constructive possession" of the SBR as soon as your stock arrives?