Might just be a section of the rib that didn't fit the bbl contour very well right there and the gap betw the rib and the bbl surface ends up being filled with solder.
They're not all perfect,,far from it no matter the mfg'r.
The breech portion is brazed together, the brazed joint around the tubes and lumps you can usually see and some take on the vertigres (sp) green color.
Some Spanish and Belgian guns will silver solder (hard solder) everything together,,ribs, lugs, ect. Those bbl's are often hot salt blued and not effected by it.
Any ting of yellow coloration in that softsoldered area might be just hardened oil or whatever has been wiped over the bbls and has dried.
Take a razor blade or other sharp thin blade and touch the seam, you can soon tell if it's lead solder or not. It can also many times tell you if there is a break in the soldered seam there if it pushes thru.
The 'ringing of the bbl's' is OK but don't bet your house on it as a 100% reliable way of telling one way or another if the soldered seams are solid or not.
It'll generally sound off a loose rib (unsoldered on both sides) at the very end/tip of the muzzle or breech, or one up against the forend lug underneath as it rattles. They vibrate like a clarinet reed.
But a simple void on one side of the rib somewhere along the way with solid solder seams on either side of it,,and a solid seam on the opposite side of the same rib,,you won't hear any difference. Nothing to rattle.
I've had 6" of rib seam open and the bbl's chime very nicely.
It's rare to have any set of tubes go into the rust blue water tank and not exhibit some small break in the solder seams somewhere. Sometimes just a small bubble leak, othertimes more than you'd care to see.
It's about 8 feet + of soft solder joint seam in a SxS bbl set. Not all the same contact area contour.
More than enough for the possibility of a bridge in the seam or a less than perfect fit resulting in a solder filler rather than a soldered seam.
Put on a magnifier and take a very close look at that suspect area. Push hard on the rib and see if it actually moves up and down.
A spot of oil along the seam is helpful as it will move in and out of the seam if it is not secure and the rib moves while pushing on it. Easy for you to see.
Resoldering an area like that is not difficult. The hardest part is getting it clean. Then just the smallest amt of flux and a thin bit of hammered out soft solder wire layed in there. Bring up the heat (indirect) and it will draw it right into the joint.
Clamp the ribs lightly along with the F/E lug. Not trying to re-position anything. Just hold orig position and add solder to the joint,,if that's what in fact it needs.