Top quality, made by Polini . This is a screw in coupling connector for fitting a fuel pump, essential to the 35+ BHP SSR Lambretta engines and 35+ BHP Malossi / Polini / Pinasco Vespa casings.
   These are usually drilled and tapped to fit somewhere on the casing where it can use suction from the crank to cause the vacuum (see second picture)
 
Pipe connector = 3.7mm Ø internal and 5.3mm Ø external
Thread = M6 x 1mm pitch