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.
![Image](http://www.simviation.com/yabbuploads/61CliffLiveSweden2-30.jpg)
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.
![Image](http://www.simviation.com/yabbuploads/59CliffLive.jpg)