There were only three designs of 4 compartment clerestory coach with a central luggage compartment. Two designs, diagrams U27 and U28, are classified as CAA12N and are described below. A third design, diagram U29, had transitional design sides and is classified as CAA7TN. There were 20 coaches built to diagram U27 and 10 to U28, all in 1883.
Distinguishing between the two diagrams can be difficult. Both were 31' long x 8' wide. The key is that U28 had symmetrical compartments and this can be seen in the width of the panels between the compartment windows. Even so it can be difficult to tell between the two designs, especially is later years when the panelling was plated over, sometimes completly, and sometimes partially.
There are several photos where a U27, sometimes in the background or after conversion to a camping coach:
100 Years of the Great Western, plate 57, 9984 ex 6144
Great Western Photographer in Wales, 49, also 9984
Great Western Autotrailers, part 1, page 123. Bottom photo
Great Western Branch Line Modelling, part 2, page 88. First coach is U27 and the third coach is a U29 (notice the transitional style sides where compartment windows are taller than the door windows).
This U27 number W9981W ex 6196, it was converted to a camping coach in the mid 1930s and now appears to be in BR all black departmental livery.
I have only seen one photo of a U28, It is in Great Western Coaches, volume 1, page 75.