Bosworth Wilson Prostaff 4.5 si Racquet In 1991, the Wilson Pro Staff 4.5 si became the next version of the Pro Staff family. It features a dual taper beam, Power Weight System (PWS) on the hoop at 3 and 9 o'clock and has a 110 square inch head size. This is a pro stock version for pro player testing and does not feature the commercial cosmetic. It weighs 11.8 ounces unstrung, is standard length and features an 18 x19 string pattern. In all black, the only model designation on this racquet is at the shaft were it reads "Wilson (Dual Taper Beam)" on one side and "Pro Staff 4.5 si" on the other side. A unique racquet from the Wilson pro room and a great addition to any racquet collection.