I think there was definitely a difference in how US studios and European Studios of the time period (mid to late '50's) derived or thought about deriving stereo.
I've also not seen many (any?) US desks from then with pans, although I believe I've seen photos of motion-picture remixing consoles with the ability. *
Some of this might be to do with the early adoption in the US of 3 track Ampex machines, at a time when the UK only had twin track - either used as stereo, or as duo-mono.
From what I understand of US facilities, the more extensive desks of the time period were 3 buss, along with there being 3 monitor speakers.
So, Nelson Riddle and the orchestra are on the left and right bus (and speakers), with Ol' Blue Eyes Frank on the centre bus and speaker.
During mix down from 3 track to 2, this was simply a passive mixing/splitting of the centre track into both left and right buses.
True?
As for the UK REDD pans, I can say that the theory involved was very much derived from the work of Alan Blumlein's work back in the '30's. This is shown in the documentation for the Sum & Difference as well as the panning laws.
There's a direct line of descent from Blumlein to REDD for sure as Len Page (head of REDD), as a young man, had worked with Blumlein at EMI until Blumlein's death during the war.
* Andy Johns once told me a funny story, at least it was funny how he told it. Which was that when he first went into a US studio to mix some Zeppelin material, he saw there where no pan-pots on the desk. He asked about it and so, with great fanfare, they proceeded to have the tech wheel in the pan-pot on a trolley. His words not mine. But you get the gist.