I'd use plain soft solder as well to repair a loose rib on the 24. One that is loose at the muzzle is the easiest to get at, clean up for soldering, tin and clamp back down & sweat solder.
Lead/Tin,,,or the newer Tin/Silver. Either is fine and melts & flows around 400+F.
Model 24 shotguns have their separate breech lug welded to the bbls.
The forend lug is often found to be welded to the bbls as well. Though some forend lugs were Hard Soldered (Silver Brazed) into place, I don't recall is these were the earlier or later ones..
It seems to me that the ribs themselves were Hard Soldered in place as well in a temp controlled furnace .
this is a 1200F+ temp range process.
Trying to do a patch repair to a rib with that sort of heat will end up presenting you with a big mess and cleanup even iff successful.,,Assuming the rest of the rib(s) are actually Hard Soldered/Silver Brazed into place as well.
All these attachment methods were to allow the bbl sets to be blued in Hot Salt bluing tanks/.
Skipping the Rust Bluing method for the bbl bluing was another of the many cost saving measures for the Model 24.
