If its a ventilated rib (which you say it is) move the sights until you shoot properly.

Slide the rear one forward and the front backward until you can shoot where you're aiming.

I've seen older shotguns where the sights are only 6 inches apart, somewhere near the middle of the barrel.

Just an afterthought, but if you can't figure it out, go to walmart and spend $30 on a cheap red-dot and spend another $25 on rings and a base. For under $60, your problem will be solved.

PS> Remove that rubber and just slide the sights until you group consistently near your point of aim. Being 2-3 inches high at 50 yards is OK---at least for me---as long as it is consistent. That rubber may/will deteriorate, stretch, or shift with the recoil, and then your entire sighting system will go to crap.