Gary D. is correct.
G&H attached the bases with tapered pins & screws then dressed them down flush w/the outside of the base & on the inside of the action rail.
They did such a nice job you need to look very closely to see where the pins & screws are but in your photo of the inside of the action rail you can faintly see them & usually you can also see them on the side of the base if you get light on it @ the correct angle because the pins & screws take the bluing a bit differently than the base.
On many of G&H bases installed outside of the G&H shop the screw heads are not dressed down & are visible.
