Chatty lied to you.
https://truewestmagazine.com/article/the-sideways-grip-cool-or-crazy/

Perhaps the earliest use on film of the so-called sideways grip, although it wasn’t referred to as the gangster grip until several decades later, was in the 1956 western movie, The Fastest Gun Alive. In it, in order to impress the local townsfolk of his speed and skill, star Glenn Ford quickly swipes his Colt across his cartridge belt, using this sideways hold, cocking the hammer against the belt as he makes his draw and fires his Colt. – Courtesy MGM –