I only made two trips to the west slope, 1997 and 1999 (and in 1995 the east slope), but was there for a week each time and never saw a single unit helper. Only pairs of SD45-2's or SD40-2's. That solo SD has real a Sand Patch vibe to it.
Yeah, the single unit helper was quite rare, and I agree, that has a Sand Patch vibe. What you saw on your visits was typical (3 and 4 unit sets were also a good bit more common than rare, but pairs were the mainstay).
Also, I want to believe that's a C&I 50 ton offset hopper at 1:24 (but I suspect it's a 3 bay 70 tonner).
And those G52Ls starting around 2:30/2:40 are just about brand spanking new.
Good stuff.