This style of coach is classified by Lewis as LA0MW. In total there were only 8 coaches built to this style and initally they formed one train. There were only 4 5 compartment coaches built to 2 designs in 1884. Remarkably I have tracked down a photo of one in a troop train. This is an S20 diagram, 5 compartment second, number 2465. Overall length 28' x 8'6" wide.
The other diagram was U18. Originally built as a 5 compartment first, the overall length was 31' and they can be distinguished from the S20s by a wider panels between the windows. In the 1890s, they were converted to composites. Then one of them had one end compartment converted to a luggage compartment.