How would you go about forging a rib ("ridge"?) along the middle of a steel strip? 

Like on this Pugio dagger, or on a feather.

There may be different techniques for a rib on one side and on both sides.

 

Top and bottom swages are one method.

It's easier with top and bottom tooling, but it can also be done half-on the anvil edge.  There is a guy in Poland that does some fantastic reproductions of Germanic spear points with central ribs.  He does all of his work with just a hammer and anvil.

I suspect the answer you don't want to hear is that you don't forge the rib, you forge everything else away from it.

Well with real wrought iron I could see forge  welding a piece down the middle to be forged into a rib...

I guess you could do that.  (Well, I probably couldn't)  You'd have to weld on the edge steel afterwards, but that would have been the norm for much of history

In episode 108 of Forged in fire, they try to recreate the jambia which has the same central ridge. Some grind away from the ridge others use a die for the press.

Power grinders and presses are modern tools. But such ridges where commonly forged 2,000 years ago.

Note that sometimes the ridges are tappered. If they used swages, this means the entire length of the ridge was formed in a single die (stroke?). Is this possible in that era's technology?

Depends on what you mean by "power grinder". If you mean, "powered by electricity", then yes, that's certainly a modern invention. However, grindstones powered by water, animal, or apprentice power have been around for a looooooong time. 

It's quite possible that a central rib could have been swaged with a constant cross-section along its entire length and then ground to shape, either with a rotating grindstone or on coarse whetstones.

Or it could be done with a series of decreasing size swages and then dressed to be an even taper.

One method I haven't seen mentioned is scraping it in with a sen type cutter like they do to make bohi on Japanese blades.

It's chiseled in. The chisel has one face flat and vertical. The opposite face is at an angle and slightly rounded. The cutting edge is slightly rounded as well in order to walk the chisel along your scribed line and not leave any dingmark's..

I'll see if I can get a pic of mine. Alas, we had a major snow yesterday, so it might be a day or so to find it.

Anvil: When you say "slightly rounded" do mean the edge has a curve similar to an axe? 

When I say or think of a "rounded" edge I'm thinking convex for toughness. 

Snow eh? Seems every time I plow and clean up the drives and shop yard it snows again. It didn't snow enough last fall to plow but it started again this morning. I don't think I'm allowed to slack this spring.

It's sure beautiful though, smooth soft, white quiet. The dachshunds don't like dragging their junk in it though. I get that. 

Frosty The Lucky.

Snows almost gone. I got nearly 18" of heavy wet snow. We needed it.

Words are tough so I'll try again 

The front face is vertical and flat. This creates the vertical side in the example.

The opposite side is at an angle and convex from side to side, not up to down. 

The cutting edge is convex 

These last two convex edge/surface make it easy to follow a scribed line and not make ding marks. They also make it a very nice tool to cut curved lines because the convex edge is easy to walk around. Also, if the tool is held at an angle(not vertical) you can smoothly make a varied reveal kinda like a champfer'd edge as you proceed around the curve whilst maintaining a vertical inside edge.

It's a handy tool.

Hope this helps.

A butcher with a curved edge?

Snows gone. I'll take some pics and post today along with some samples of it's usage.