Durifort is a product of the same company that manufactures Vitus (if memory serves this is Ateliers de la Rive or something like that). A step up from gaspipe is about right. I came into possession of a bunch, and the main tubes, which are stamped Durifort 888, appear seamless, with plainguage walls of about 1 mm thickness. The fork blades and seatstays, stamped Durifort, have a visible seam. Gitane Interclubs claimed to be made of this stuff (then again, they used to claim The Gran Sport DeLuxe was seamless- I was a disappointed 14 year old when I stripped mine to paint it).
Regards,
Wes Gadd