I have two of them and I use them on a pair of pre 1930 Hand Ejectors. They shoot like target pistols with those old half-moon blades settled in the notch of the Wondersight.

It takes a bit of doing to get the elevation dialed-in, but it's worth the trouble.

The Wondersight doesn't mar the finish at all, and can be removed and replaced with the original bug screw in a few seconds.

There's no point in using the Wondersight if you're not going to shoot out past 25 yards. Mine are zeroed in at 50.