Thx.
Unfortunately this is not the typical Gios seat stay in the '70's. But I'm not a Gios expert.
It could be an earlier model or maybe a cheaper type. In any case, the blue paint residue in the lugs also points to the direction of Gios.
I see you live in Italy, maybe if you are in contact with the Gios factory, they could give them the exact answer.

Of course, it may also be that another manufacturer has copied this solution on the BB shell.