At least in the old days, nice smooth, rounded fillets on fillet-brazed frames were not all due to brazing talent. As mentioned in Mike Rother's article "Whole 'Lotta Brazing Going On--The Story of Schwinn Fillet-Brazed Lightweight Bicycles--1938-1978," on Sheldon Brown's web site, at least on the early fillet-brazed models, lead was used to smooth the fillets. This was similer to the then common practice of using lead to fill and smooth automotive body panel joints. I saw the leading first-hand when I stripped the finish from my '38 Schwinn Superior. It was apparent that the head and upper seatpost joints received the most smoothing attention--virtually no brass could be seen--only gray lead; the bottom bracket joint is not nearly as nice and showed little or no leading.
Pete Rutledge
Woodbridge, VA