Neat looking rib.
Late to the thread, but I'd offer that the rib is a solid piece of steel. Silver soldered (hard soldered) to the bbl. Done with the bbl removed from the frame of course. The slight gap betw the back end of the rib and the bbl allowed a bit of the silver solder to show afterwards.

I'd bet the fit of the inside curve of that extremely thin portionof the back end of the rib to the round bbl wasn't exactly a premier fit. The extra hard solder in that area helps support the very thin steel rib there as well as attach it.
Silver solder will stand up to hot salt blue just fine (if that's what they used),,soft lead/tin solder will degrade from it and loose it's strength after a time.
No problems with rust blue of course with either soft or hard solder.

Silver (hard) solder won't take bluing,,either hot salt or rust blue. It stays it's yellow color and will depend on the alloy of solder used of which there are hundreds.
If the bluing is rust blue, the carding wheel operation necessary after each cycle would give the tiny bit of hard solder exposed a grainy, matted appearance,

Doubt if it's brass/brazed on, that'd be way over kill in the heat dept nearing welding temps.

The cross hatch marks on the top of the rib are all hand cut,,hammer and chisel. No machine work there.
Not a big deal for most any engraver and fairly fast to do. The chisel cut cross hatching would have been done after the rib was attached to the bbl. It would have needed either a perfect supporting fit at that thin back end,,or what I suspect is that extra layer of hard solder built up underneath that area that you see exposed slightly. Or else the surface can cave in on you.
Something you run into doing initial shields inlayed into gunstocks occaasionally. You never know how thick the metal is or how it's backed up unless you've done the install yourself.

There's a tiny faint 'witness mark' on the frame top front edge, right side of the back end of the rib (first pic). That was used during fitting up the rib, centering it at 12 o'clock and would have probably been used when reinstalling the bbl back in the frame when done.