See Brownell's. Hi-Force 44 Solder (475 deg.) and copper bond flux is probably best for the job. I suspect any silver bearing solder (about 4 - 5%) and recommended flux would also be pretty good.
But it is VERY difficult to solder on a rib. Not a job I'd try. The surfaces have to be absolutely clean, the rib clamped on but it and the barrels can't be warped, and you have to sweat it, heat it with a torch and let the solder flow in, enough to fill the joint but not ooze out. Unless you're an expert it'll mess up the finish.
Too technical, Silver Solder is Out, Too much heat is needed, I have shit loads of 6B Pencils to coat the bits I don't want solder on.(Graphite jjk308) (its a Shooter not a Collectable). Am Actually looking for an original Flux used then, not a Flux available now.
Might do the job. Problem is a lot of solder has all the lead taken out so it doesn't flow well. A small percentage of silver in the tin seems to help a lot with that, makes it easier to use on guns, and I use it exclusively when plumbing, too.
I've never tried Brownell's low temp stuff but a brand of low melting temp solder I tried once was very weak.
An original flux used then was powdered rosin. An athletics store usually has it in small quantitys for tennis players, baseball players etc. Another was 'Salammoniac' (Ammonium Chloride) which you can still get in block form (1lb blocks usually @ $6) at most any welding supply store as it is still used as a soft soldering flux. The fumes are something else from this stuff and they will coat and rust most everything in sight, so good ventilation is a must with it. It was also used as a rusting agent for browning the metal work on early muzzle loaders.
When someone speaks of 'silver bearing' solder, it IS soft solder with silver alloyed in it to make it have a lower melting point than plain lead/tin soft solder. It melts at around 350/400F depending on the brand. The Brownells 'Force 44' brand mentioned is one of the most often used in the business and a good one. It works with most any flux. There are many 'no-lead' soft solders out now because we all have to 'go green' don't you know. Most are nothing more than the old bismuth solders and though they work at extremely low temps (200F-300F) they have little strength for this type of work
You're right that regular cadium based 'silver solder' would need much to much heat to get the job done as even the lo-temp stuff needs just under 1000F to flow.
Personally, to re-attach a rib I'd get it nice and clean, and use nothing more than standard No-Corrode paste flux and standard plumbers solder. Nothing fancy but not much to go wrong either. Plenty strong. The lead/tin solders will turn grey and age down after a short while if any lines of it show at the edges of the joint. The silver bearing soft solders are forever bright white.
Ditto on ktr.
I have most of a 5 Lb. spool of old 50-50 solid core solder I use with Brownell's liquid flux to reattach shotgun ribs. The old gunsmithing book by Dunlap has formulas for various fluxes. If you cannot work it out, get back to me and I will look them up for you.