Go back to the comments by Kuduae. I believe he mentioned some automatic function, involving the set trigger. The threaded hole is likely for the set trigger adjustment screw. This might be the source of your problem.
I see what you mean, I don't exactly agree with his interpretation of the patent because the selector lever is clearly there on the tang in the drawing. Part of my problem is not even knowing if I have a problem! lol
The action functions, if you select the rifle barrel on the tang, it fires the rifle and the tang selector resets. But in order to fire the right shotgun barrel again, you have to press the button in the trigger guard. You could keep setting and firing the rifle barrel forever, but until you press that button, you can't fire the right shot barrel.
It just seems to be an odd little routine that I can't figure the reason for. It seems like an auto reset to the shot barrel makes more sense, but then that would negate the need for the button?
I need to talk to someone who has the button in the trigger guard and ask how theirs works. Thanks for the input Der ami.