It appears that you're lucky it lasted this long. There was very little good clean solder joint holding that forearm lug and short section of lower rib. If there had been a good solder joint to begin with, water could not get under it to cause any rust formation. I've seen lugs and ribs that popped loose that had more shiny solder contact area than this one. If the solder joints were this bad in this area, I'd tend to think that there are plenty of other voids, gaps, and pinholes in the remaining ribs. Probably caused by poor cleaning and fluxing during soldering. A complete relay might be in order if the gun is valuable enough to make it worthwhile.
