There is a class of solder which in the machinist trade we always referred to as Silver Solder. These had a melting temp lower than Brazing but well above that of soft solder. One example of these & was a very popular one where I worked was Eazy-Flo 40 if recall correctly. It had a flowing tamp of around 1145° F. It had a much higher tensile than even Force 44 or the Eutectic solder we used. I forget the actual number but it was lead-free & had been designed specifically for the Dairy industry for soldering stainless tubing. Its flow temp was very close to that of Force 44. By being Eutectic that meant that all of its components melted together so you did not go through a "Pastre Phase". Eutectic solders are the easiest type to work with due to this feature.
I am not certain that this category of solder had been developed when that NID was built. Sure looks like a mighty short contact area to me for a soft solder joint to hold up well. I measured a couple of my guns, a J P Clabrough & a Lefever & both had that rib section between 1˝" & 1 5/8". Both are Brazed. These Should Hold till the Cows Come Home.
