These guys don't know much about what they have. For one thing they have their railroads reversed.
Sumpter Valley has two of these cars, one of which came from the WP&YR, and the capacity is 6500 gal. and the brake valves are K-1A. By ET-6 I presume they are refering to the 6-ET which is, of course, a locomotive brake system. We actually have a third car with the same tank but different frame and type AB brake valves.
As most of you know, all of these narrow frame cars came originally from the D&RGW. One of them was sold to the original Sumpter Valley Ry. and has been in Oregon ever since. We do not have it restored yet.