First, I must make the distinction between "bikers" and "motorcyclists". While both are surely rebel characters in many respects, the latter of the two should be distinguished for his elegant sportsmanship. This "distinct breed" of gent was an athlete whose thirst for danger and competition set him apart. When talking specifically about motorcyclist jackets, such as the Trialmaster and who wore them, we are talking about motorcyclists.
Next, it should be noted that when I made the assertion that Barbour was for "the hunter and the outdoorsman" I failed to mention their own motorcyclist jacket, the International. In fact, in the iconic picture (see below) of Steve McQueen at the 1964 International 6 Day Trials he wears a Barbour International and not a Belstaff Trialmaster. Despite this documentation of McQueen in Barbour his feelings on Belstaff were actually quite fond.
It has been said that despite McQueen wearing the International at the '64 Trials in Germany, he actually preferred a Belstaff and wore that whenever he, himself, biked. In 2007, Belstaff even released a special edition Trialmaster (see below) as a "Steve McQueen Celebration."
I hope that sufficiently takes care of everything and serves as a kind of mini history lesson. Thanks to all the readers who helped me get my facts straight and who participated in the initial discussion. When Sart Inc is "getting its forum on" I couldn't be happier.