John, the key guard feet would have been braised/hard soldered ( silver solder) onto the wire guard on your sax and the the guard would have been soft soldered, onto your sax body,using probably 60/40 lead/tin or something similar.
Generally a solder with a higher melting point will produce a stronger join. As with any solder joint the good recipe for success is a tight fit of both parts, spotlessly cleaned, degreased and fluxed. Followed by a thorough clean with a mild soapy water. For the best results I find that once all the preparation above is done and the two parts are held together securely, I cut a small piece of solder e.g. 5mm long for soldering a key post onto the body, then place it along the joint, add a little more flux using an oiling bottle with a needle dispenser on the end, heat the area, not just where the solder is, and wait until the solder flows. If the joints are clean the solder will run along the flux toward the heat. If I need more solder then I repeat the process. I prefer this method to the "feeding method"(where the roll of solder is fed onto the joint) as it saves on clean up time (excess solder etc).
Always bear in mind when soldering on instruments to remove any keys/pads/felts/cork in the vicinity or mask it from the heat using a heat shield ( I occasionally use a small soldering blanket or layers if tin foil) and any parts that are soldered onto the body are appropriately secured using wire and soldering clips.
