Jason,
Are you sure it isn't a model 120? The device that blocks the magazine tube so it can't be loaded with more that 2 rounds is called a plug. I can't actually see a plug in your pic, but you should be able to see a plastic rod that moves freely within the tube. To remove it, remove the magazine cap/barrel nut and pry gently on the retainer while holding a folded towel over the whole assembly. If you pry too forcefully, it will deform. The retainer will pop out, the spring will partially escape into the towel (or air or into your face if precautions have not been taken) and the plug will simply drop into your hand.
Caution is needed for this operation, though! The retainer is under considerable spring pressure! When the retainer comes loose, be absolutely certain what it and the spring will strike. A folded towel is greatly preferred here. Two of the negative consequenses to be avoided are having the spring strike you in the face

- which WILL leave a mark ( at the very least!), and part or parts can easily sail away & not be found again! After the plug has been removed, compress the spring back into the magazine tube and carefully squeeze the retainer back into the end as it was before you removed it.
Keep the plug. It is illegal to hunt certain game species without it.
You can shoot steel shot in it, but I'm not 100% certain of slugs in a full choke as I have never fired that combination. A call to a full-service sporting goods store in your area will clear that question up for you.
I can't think of other comments concerning the model 120, but do be cautious when prying the retainer from the magazine tube. SW