Doc, I can't really tell anything from that photo; but if that trigger is a Hunter One-trigger, here is my guess. The original trigger shoe seems to have been removed and replaced by a shoe with a longer and more sweeping curve. If that's the case, I suspect the knob is a device to prevent the trigger from traveling, or being pulled too hard against the trigger plate. These things won't function without a tiny bit of let-off; so maybe, without that little bit of jury rigging, the modified shoe allowed the shooter to pull the trigger too far so that it would not properly shift to the second barrel. Your guess is as good as mine; but without the stock off the action to expose the mechanism, who knows?