Some sort of old school Trijicon or even LPA sights. Not factory Remington. Strange how it seems the rear sight is replaceable from the base by forward or rearward movement (rather than traditional left to right dovetail). You wouldn’t be able to adjust for windage if they weren’t installed true to bore.
I like to install shotgun sights (especially soldered rifle sights) perpendicular to the flat sides of the receiver using precision levels and squares. The flat part of a barrel lug would be great as Dennis said, but it is a tight spot to reference for most and sometimes manufacturers tolerances will make it seem slightly canted. Using the top flat channel on receivers can be a guessing game at best, especially with Remingtons, or guns that have been refinished. The front of the channel will be level, but as you move backward it will change one way or the other. Then you need to ask yourself, which section is true? Who knows…
Taylor C.