The photo at the top made me think about GWR diagram numbers. Why? You might ask. It is because of their diagrams
Diagram O8
Diagram O8
How's that? The Tube (or Open C) wagon is in the wagon index and the Siphon C is in the Passenger index.
Each index has letters and numbers. The letter is a category or vehicle and the number is just a unique identifier. The GWR set these indices up in early 20th century. Generally, the higher the number,
