I'm not surprised that it doesn't show up in the 1930s-My impression is that it was likely a post WWII structure, and it does appear in a 1949 photograph in Myricks' Railroads of Nevada and Eastern California.
I agree that it would not be ideal for unloaded ore as it appears to be a little low to unload into a gondola by dumptruck.I didn't notice any spilled ore along the sides.
On the other hand it does not seem ideal for unloading equipment either-It seems like it would be difficult to get equipment or vehicles off a narrow gauge flat car at a 90 degree angle...but the heavily hinged ramp at the end of the loading dock seems to support this theory.
and a bonus pic of modules (under construction)being photographed en situ at Keeler:
