In 1993 I had a Pesenti-built frame, EL/OS tubing, tigged, and a bit similar to yours. EL/OS is a Nivacrom tubing. Columbus introduced Nivacrom to be able to tig-weld the material, instead of brazing. I think they developed this mainly for the more cost-effective construction of frames it made possible.

The EL/OS frame was a great ride: definitevly in my top-three of bikes I rode and raced. The very thinwalled and slightly oversized tubes were really wonderful. Only problem: the seatstays cracked (both) between brake bridge and seatcollar. Im not sure what the reason was, maybe they still had to learn how to tig-weld thinwalled tubing, without building up too much heat in the wrong places.

So maybe your Rossin is EL/OS! Check the diameters of the tubes ..