[mrerick]

The rules don't make particular sense, and unfortunately they don't have to.

Note that it may be considered "constructive possession" to have an attachable stock, have an incorrect Luger (one that was not released to go with that specific stock type) and not have the correct Luger that it goes with.

These kind of things may not be actively enforced, but you're exposed should you ever have an incident that leads to investigation or to a change in government leadership that leads to aggressive enforcement actions.