M1S90T

Just picked up a mint M1 Super 90, HK import, 18.5 with a threaded barrel. Are these Mobil choke guns? The choke that was in it is improved cylinder, steel shot ok, with four **** after it. I'm assuming this is a factory threaded barrel. Previous owner claims the gun was imported with a pistol grip and 7 round tube. Where can I find a replacement spring for the magazine tube?