In reality, the USMC was not really interested in a "plus" upgrade at all, since the cost was prohibitive, they jumped in the wagon since Spain and Italy were interested in a multipurpose aircraft for their respective navies, and the whole project was the only way to get such an aircraft, given the numbers and shared risk between the 3, the "plus" became a reality. UK was not interested since back then they already had the SHAR F.2