I have one of those Peugeots, which I bought new in 1999, with Columbus tubes. I really like those invisible joins. More recently I found something about how they made it and it was pretty repeatable. It relies on very accurate mitring (which is done by a machine) and then it's hearth-brazed. The idea was high reliability and low-cost for mass-production. It all became academic as a few years later everything moved to the far east and mostly became aluminium.

My Peug is very well-aligned and hasn't fallen apart after main thousands of km. The only criticism I have of the build is the bottle cage holes in the ST are clocked slightly to one side.