This style of coach is classified by Lewis as LA0MW. In total there were only 8 coaches built to this style and they initially formed one train. There were only 2 6 compartment coaches built in 1884 (the only 6 compartment short coaches built by the GWR). They were given diagram S21 and were 31' x 8'6". Remarkably I have tracked down two photos. The first is of a train at Hannington. The second nearest coach with its door open is a later Metro design with elliptical roof and a 4" eaves panel above the windows, Lewis classification LE4MW. The fifth coach is an S21.
A second photo of an S21 coach is in Great Western Stations, Volume 2 Wales by C Judge. Plate 19 at Blaenau Ffestiniog. It is the second complete coach from the right hand side.