Excellent vision aid - Burris SpeedBead Red Dot Reflex Sight

As a Skeet instructor I have students with a variety of visual challenges. Many are cross eye dominant. Usually taping a lens takes care of this but not always. Sometimes a student shoots 20 feet in front of the target on crossing shots no matter what teaching approach you take. I put this on my Beretta 390. With it in place, shooters can see where they are pointing much better than without it. They can compare what they see with a picture or video and learn the correct sight picture. I then go to another gun without the SpeedBead and the new skill transfers.
For skeet shooters I have it adjusted to deliver a 60/40 pattern. When I want to shoot Trap, a half turn of the elevation dial gets me to 70/30. I don't need to switch guns or raise my comb.
They don't make a plate for the 390 but they have one for the 391. It took quite a bit of modification to get it to fit my 390. In addition to some grinding to adjust the shape, the mounts adjustment screw was not long enough to bring the dot down far enough on this gun. I cut a short section, about 1/4 inch, off of the end of a 1/8 inch drill, dropped it into the hole for the adjustment screw, put the screw in on top and that did the trick.
You will see reviews that complain about the fit on any particular gun but if you have a Dremel tool and a brain you can get it to fit.