I always referred to that type of rib configuration as a "Stepped Rib" as apposed to a "Flat Rib"
Beretta 390s and 391s were made in both configuration.
As far as the front bead, I can't see how it would not sit on the same plane as the top of the rib. If it sat on the same sighting plane as the top of the receiver, it would be obscured by the difference in height of the step rib.
Even with a mid bead, the only way to see both beads is by looking down the rib, not the receiver Or if you are using a figure 8 to stack the beads, you'd be looking down on the top of the rib.