I also haven't seen such joints on a steel bike before. Is it because of the bigger diameter tubeset? As far as I understand, we are dealing with a frame that is both welded and fillet brazed. Is this correct?
From what I can see in the pictures of (Mark) sinnett
's bike, all the main joints are fillet brazed. Sometimes brazed joints are tack-welded before brazing, and occasionally joints are welded completely, then a fillet of brass is applied over the weld. I can't tell if that's the case here.
When Columbus Max was first released, Columbus produced a set of special shaped lugs to go with it, like the ones on this Rossin:
They were heavy, and restricted builders to a narrow set of geometries, so it was common to either fillet-braze or TIG-weld Max frames. I don't know why Gitane chose to fillet-braze their Max frames rather than TIG them. Often smaller builders that specialize in lugged frames will make lugless bikes with fillet brazing because they already have the necessary equipment, but a major manufacturer like Gitane would certainly have had access to TIG welding equipment.
Just for comparison, here's a TIG welded Columbus Max Kona Explosif:
http://i779.photobucket.com/albums/yy73 ... G_1052.jpg
and a fillet-brazed Columbus Max Santana Moda:
http://gallery.mtbr.com/data/mtbr/507/2 ... CS_018.jpg
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.