thank you Larry for the help,
I tested distance between the fixing screw hole and the seat end side; it is the same as the plastic tank...
my problem is to know if it's a original Bultaco's Sherpa tank or if it's for other models or a replacement built here in Italy.
It's because i never seen, live or in iternet pictures, a Sherpa tank with this hump between high front end and tank cap :-)