There are several styles of clerestory as defined by John Lewis. However, there was only one design of 4 compartment clerestory coach and it is classified as CAA9N. There were 30 coaches built to this design in 1877 and it was allocated diagram U10. Initially the central pair of compartments were for first class travel and the outer pair were for second class travel. It was 28' long x 8' wide and apparently built on old Broad Gauge chassis, which probably accounts for the unusual feature that the ends are flat. There are several photos where a U10 appears in the background:
A Pictorial Record of Great Western Engines, volume 1 figure 119
Devon & Cornwall in Old Photos page 135
Severn & Wye Railway, volume 1, page 127
100 years of the Great Western, plate 163 - this is probably the best photo
This is my drawing of this design based upon information in the Lot Book at York.