Shelby was well known as a hustler with little thought for anything beyond making a buck. He came from relatively humble beginnings, which probably colored his thinking. He was an excellent road-racing and Formula 1 driver, but was forced to retire due to a heart problem. He was known to be willing to license his name to most anything if it would make money for him.(He was known to have said that the tiddler engine in the 'GLH-S' Dodge cars with his name on them would soon take over as the most ubiquitous engine for performance cars'.)
The holster type shown is often called a 'Yaqui slide', and, if I remember correctly, came out of the group that loosely gathered around Jeff Cooper, practicing what developed as the way all users now handle handguns, shortly after the Second World War. (See 'Cooper on Handguns')
KS