Shimano's U-brake didn't come out until the 1987 model year, so it's should be no older than that, assuming they are OEM. The fad for chain stay mounted U-brakes quickly ran it's course, so it's unlikely to be other than very late 1980s. As noted, Sintesi was a Bikeolgy house brand, along with Supergo. The serial number may allow us to determine the actual years and manufacturer.