Are you sure this system was smoothbore? I seem to remember many years ago seeing pictures of the muzzles of these tubes and they were rifled, but I could be mistaken on that, I admit.
I would have said also that spin stabilisation was achieve far easier without fins, than with them, because of the drag associated and the time required for them to flip out, after the rocket has left the tube. Most spin-stabilised rockets utilise exhaust gases to start the rocket spinning, even in the tube.
Either way, why such a long tube? I suspect you'll find that the rocket fills only a third to half the length of the tube, not its whole length, to enable the rocket to "spin up" before exiting the muzzle of the tube and gaining some level of gyroscopic stability.