They have clearly been rebuilt, as those large viewing windows are not original. The roof shape is the same, except on the combine #211.
These, to me at least, are very unusual US passenger cars in that, unlike most other cars of their era, they have no clerestory roofs. The only other cars that I can think of which had no clerestory roofs, on the narrow gauge, were the very early 2-axle Denver and Rio Grande Railroad cars and the Colorado Central/Argentine Central Railroad Excursion Cars.
And more recently, the first Cumbres & Toltec passenger cars which were built on D&RGW Flat Cars, and which are still in use on the C&TS.
Is the appearance of the Sumpter Valley cars just down to the design used by their manufacturer, or is there another reason?
Chris