It’s not fillet brazed. Schwinn used a fast industrial method to put the front of their bikes together. A two piece lug like part was fitted, clamped tightly in place and high voltage was applied, turning the whole thing white hot instantly. It’s called electro forging. I’ve been in a factory that made wrenches like this. A huge spool of thick wire fed a wrench length into a clamp, high voltage made it instantly white hot, a stamp came down and smashed the thick wire into a wrench and it was snipped off the wire, a wrench every second. This was shipped to another factory for finishing and plating. If you see a frame like this it was made in Chicago.