It looks like Silver Braze, Perhaps Bag-7 ? Pre-form filler wires are an excellent way to control the amount of filler in the joint. A reducing atmosphere, a positive pressure Hydrogen belt furnace would display this type of look on the finshed product. Its definately not torch brazed with flux, otherwise you would see a strong display of the copper coller caused from the flux. As mentioned different fillers with decreasing melting temps are an aid to multi-joint assemblies.
FYI, Brazing is above 840° F, Soldering is below that. Alloying ingredients control this transition stage as well as joint properties.