Rich, judging by John's photo posted here and by others, Los Pinos tank wasn't a 50k version back in the day either. Note that both the new tank and the old one only had 3 support bents vs the the typical 4 of a 50k version. I'd also guess that the one pictured in the 50s or 60s wasn't the original tank either, as it seems most of the tanks along the right away were replaced between 1910 or so and the 40's. Some were replaced with a newer wood tank, others such as Sublette and Cumbres replaced with cisterns. It is very possible that it may have been 50k to start with, but shrunk at some point during a replacement in Rio Grande days.
If I had to guess, when the K locomotives came on the scene Los Pinos was there for emergencies only and to water rotary plows in the winter. With a big tank in Osier and mostly level track from Osier to Los Pinos siding, would a loaded freight need to take water at Los Pinos??
I'm sure someone here knows more than me, though, so I hope we here the same story. I've always been curious as well.