Great detective work, Dave!
Horseshoe makes sense as a location for a water tank, as the steepest grades were to the west.
There are two very annoying things about our favorite little narrow gauge railroad:
1. There is a huge void in documentary information about the C&S and its predecessors.
2. The documentary information that did survive very often is contradictory.
Take the Alma water tank. I rechecked my set of 1918 ICC maps and, sure enough, no water tank is indicated at Alma.
But the 1922 Employee's timetable indicates water is available at Alma. And the January, 1929, DWB Special photo posted above shows the water tank still standing, looks to be in working order.
Anyway, this has been a great thread, thanks Jeff, for posting the photo of "Horseshoe Tank".
Jim Courtney
Poulsbo, WA