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I seem to recall that 'Dale didn't used to file their welds - they welded them differently, something to do with heating the weld up more (or after it was completed) so that the material all sort of flowed into each other... I'm sure that description isn't quite right, but remember it was quite interesting at the time.

Yup you're right.
Welded differently, but definately not fillet brazed. :)

"And now a word about Cannondale's hallmark smooth weld bead. It's a common misconception that said weld bead requires sanding to achieve its smooth finish. In fact, the weld bead is smooth, right out of the weld booth. The smooth finish is created by a two-pass, puddle welding technique that Cannondale employs. Cannondale points out that their welding technique produces better penetration than welds that exhibit the typical "stack of dimes" appearance. Better penetration results in a stronger joint. Furthermore, the high and low spots on stack of dimes weld beads may act as stress risers—a potential compromise of weld durability."