I know the scenes you refer to because I saved screenshots from them a year or two back.As you know there is also a very grainy crop of a photo showing a Mk.III Ross with a severely cut down stock and a German
-made scope mounted.
Perhaps the Red Army was experimenting at that point and decided to use a non-standard rifle for the same sort of reasons that the Britishchose to use Springfields for a comparative test of certain scope mounts: apparently it was felt that choosing a totally unfamiliar rifle would focus the user's attention on the components that were to be compared.
Or perhaps it was just an economy measure, or possibly they recognized the superior accuracy of the Ross - and perhaps the speed of loading - and decided that since it was a limited run, they might as well use the best they could get?
Have you seen any reference to rifles being supplied to the Whites with scopes fitted? I never have and what's in the movie is just in a movie. Sort of like the Ross .280 takedown that Clint Eastwood handles in "Joe Kidd" (scope on backwards etc.)
If there was any intention to supply the Whites with such rifles by spring of 1919 there were about 8000 scoped SMLE's sitting in store at Weedon or Enfield that could have been sent, and since so many SMLE's were sent it would have made sense to send the same rifle, despite its many flaws as a sniping rifle. Particularly since the rifles were going to be broken up for spares anyway, though off-hand I don't recall when that decision was made.
1926 is pretty early for a scope acquisition by the Soviets; within a few years of their cooperation with the Reichswehr beginning.
I see Gen. Morozoff/Morozov was speaking to the N.Y. Times in 1942. I suggest he was being both polite and deceptive: implying the Soviets were just learning and new to all this etc. etc.!