Schwinn built tens of thousands of fillet-brazed frames between 1938 and 1978, and used straight gauge 4130 chromoly tubing with pressed-in reinforcing sleeves at the joints to act as butting. The finished frames were strong and durable, and relatively light (my 61cm Super Sport frame weighs 7 pounds without headset, BB, or kickstand). The wall thickness of the tubing measures a hair over 1mm using a caliper.