That should be a Webley & Scott 500 series shotgun, based on the year it was made. But it could be a 400. My reference shows that gun made around 1920/1921, but since you have a 1925 proof mark, we'll have to go with that.
Can you attach some photos showing both sides of the gun?
I'm just not able to match those three pins to a model with my references (they have two pins and a screw, which is nearest the hand guard.
I'm going to opt out and let another expert see if he can find the exact model.