I have run across 3 definitions of swamped rib in the last year or so, but I believe Ted's definition is the most useful, but Darnes have another rather unique way of being "swamped" according to some dealers - that is the bead is below the top of the barrels.

Concave from front to rear would be my definition, regardless of concavity from port to starboard or in respect to the height of the bead relative to the tops of the barrel.

Hallowell is usually my go-to for firearm definitions, but in this case the example seems to be simply a concave rib that may or may not be swamped.

_________
BrentD, (Professor - just for Stan)
=>/

[Linked Image from i.imgur.com]