Maybe I got it wrong then. I never took acting lessons.
Going back to the main question - I can only speak for the UK but The Shadows were the first conventional 3 guitar & drums group in this country, They were obviously a great influence on many budding British musicians like me (& The Beatles) who went on to form their own groups. I found this early photo of Cliff Richard & the Shadows perfoming live in Sweden in 1961. Note that Jet Harris the bass player is in the centre with Hank Marvin the lead guitarist on his left.

It seems natural for up & coming groups to copy this layout. As I previously mentioned, the setup would often depend on the venue. My brother played bass & I was drummer in most groups I was in. I distinctly remember him being sometimes on my left & sometimes on my right. Personally, I much preferred it when he was on my left or "stage left" as you refer to it. As I recall, this was our usual setup.
PS. Purely as a matter of interest this was taken in 1959 - before Jet Harris (with blond hair) imported his prized Fender Precision bass guitar, the first example ever seen in this country.
