The Diagrams Book of 1904 has a diagram of a prototype sleeping van. Up to this time sleeping vans were "hand-me-down" passenger cars altered with bunks and kitchen facilities.
The diagram shows vehicle "WS 1A". This is/was extremely unusual: the use of an alphabetical letter with a number to describe a vehicle. This made two No.1 _WS_ cars: number "1" and number "1A". This puzzled the writer for twenty years and efforts to resolve an answer failed. Only in recent years has the mystery seemed to be been cleared up.
A prototype van was built in 1901. It appears to have been constructed as WS 92. However, it seems likely that as it was a new type of van, efforts were made to give it a unique code and number to segregate it from older stock. The code given was AWS and it was number 1. This vehicle was later coded to _W__ 1.
From the records it was shown that the "1" and "A" were transposed to give an identity "WS 1A. What is not known is whether it is a mistake or intentionally done for Diagram Book layout. The Rolling Stock Registers have a page reference for a "AWS 1".
Between 1906 and 1909, the workshops rebuilt many of the older carriages coded WS to this new style.
Source: 1886 Registers, 1904 Diagrams Book