Another possibility, given that they have been re-blued, is that the ribs may have already been re-laid by someone who didn't get the alignment quite right. Getting the two sections of top rib to line up perfectly is one of the most challenging parts of the job, and an inexperienced hand may have trouble getting it right. If that is the case, it is still theoretically possible that the rib is solidly attached, just a little misaligned - which of itself wouldn't hurt anything.
If close inspection with a magnifying glass doesn't reveal any gaps between the ribs and barrels, and a .001 feeler gauge won't slip in anywhere, another way to find a loose rib is to submerge it in a shallow tank of near-boiling water (just like when you are rust-blueing). The air inside the space between the ribs and barrels gets heated quickly and expands. If there is a leak, the air will come streaming out in a trail of bubbles where the leak or loose section is. This only lasts a few seconds because as soon as the air is well heated it stops expanding and the bubbles stop. If you see bubbles coming out from the rib, remove the barrels from the tank quickly because when the air stops coming out, water starts going in (which is not a major problem, but best to avoid if possible). If water gets in, you can dry them out in an oven on low heat.
Jim
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