How ridiculous to have to go through so much hassle for a 9mm chattergun. Those things probably cost $50 to make, as was said, yet you paid who knows how much for a transferrable one.
At any rate, enjoy your purchase. Get a better stock on it! IMA has all sorts of STEN accessories, including mags, 50 round Lanchester mags, pistol grips, wooden buttstocks, loop stocks, etc.