Couple things I wanted to add, hit Submit too soon.

Peugeot and Motobecane both used internal fillet brazing, I think in the '80s, maybe starting late '70s? Motobecane called their process "inexternal".

Some Peugeots I have seen somehow managed to not let any of the filler metal get out -- close to zero external fillet. Looks scary to me, though I assume they were strong enough.

The other thing I wanted to mention was that unlike Jamie Swan's preforms (see the flickr link in my last post), which were a single ring, I recommend multiple loops of thinner filler wire, formed into a sort of spring. This gives multiple advantages.

Think about how a ring touches the inside of the tube in a circular line with close to zero thickness, and that's if the ring is in a plane perpendicular to the tube axis. The ring can rotate, like a butterfly valve in a carburetor, to where it's no longer touching in a circular line, just touching in two places that are close to mathematical points, almost zero area of contact. A "spring" is more like a cylinder, can't rotate out perpendicular like a single-wind ring can. Even just two windings is usually enough to prevent rotating out of plane.

Plus a spring has more contact with the tube, for faster heating by conduction, and each wind has less thermal mass "behind" that contact area, so it melts faster.

The springs need to be wound to a diameter that doesn't quite fit into the tube in their relaxed shape -- they need springing down while inserting, so they're tightly pressed against the inside of the tube.

I somewhat doubt Peugeot and Moto used springs like I describe; their preforms might have been something closer to the final fillet shape. But unless someone who worked there back then chimes in, we'll probably never know. That info was proprietary and I doubt they gave much away in public documents or ads. I'd be glad to be wrong though, anybody got more info from back then? Patent application? (Though I doubt this was patentable, being such an ancient method.)

Anyway I think those bikes are a bit ugly, so this is not something I want to emulate. Jamie Swan also said he would do a fillet pass over those joints whether they needed it or not, for the aesthetic look people expect in a lugless joint.

Mark B