Before screw-in chokes, adjustable chokes were popular on pumps and auto-loaders. If you consider the manufacturing process of a shotgun barrel, it obvious that having the adjustable choke as an integral part of the barrel would be a major bother. If you look at your second pic, there seems to be a joint between the barrel and the choke. The easiest/cheapest way to attach a choke at the factory is by sweating it on (soldering). I'm assuming this is a factory choke.
That being said, look at the porting cuts, you'll see that your gun has a relatively thick barrel. Your choke might be an after market screw-on; but, most screw-on devices have flats, or other means, to accommodate the attachment tool. I don't see any on your gun's choke base. I'm still guessing it's a solder job.
