[Jonty30]

Here are other movies.
The fact that it came out in the 1960’s would suggest some military connection, since many of the actors of this time period did serve in WWII and the Korean war.

https://www.pagunblog.com/2009/12/15/origins-of-the-sideways-grip/

[Retrofitted]

"Holding a weapon sideways has long been equated with risky and indiscriminate shooting. For instance, in the 1894 American novel John March, Southerner, by George Washington Cable, a character orates, "No man shall come around here aiming his gun sideways; endangering the throngs of casual bystanders!"

Now, that came from Wikipedia but the source checks out. So... probably predating 1894?

[higgmeister]

I noticed this in the comments of your link.

Jdudesays:

Dec 18, 2009 at 6:17 am

I’ve tried the sideways grip. I have all of my new shooters try it once as well, to prove that it is a waste of time and ammo. Nobody I know has successfully or consistently hit a 10 yard target that way.