In 1980 diagram numbers were revised into a numbering system that is often called 'Design Codes'. This involved two letters and three numbers, and an optional number/letter pairing. These followed a more logicical pattern than the original diagram 'pages'.
The first letter for DMU vehicles was 'D'. The second letter defined the vehicle type. The following two letter codes were used:
Following the two letter code were three numbers. The first number defined the passenger accomodation carried. 1xx were first class vehicles, 2xx second class, 3xx composites, and 5xx was for non-passenger vehicles such as the parcel vans.
The image shows the data carried on the rear of Class 108 DMBS (taken at Wigan North Western in March
1982 by S. Gaskell). The code - DQ 212 - can be broken down as:
D - Diesel Multiple Unit vehicle
Q - Driving motor vehicle with brake van
2 - The passenger accomodation is second class
12 - It's individual diagram in the DQ2xx range
The new pages were double sided, the front had the drawings from the original diagrams reproduced with new text (some newer versions had computer generated vehicle drawings), the rear carried more detailed information about items such as engine, gangway, buffer and coupling type. Unlike the earlier diagrams there were no reference to lot or vehicle numbers for each design code.
There was a reference back to the original diagram, and for DQ 212 this was BR 543. Although the vehicle in the photograph is unknown, BR 543 covered 50599-50629 and 50924-50987 - the DMBSs with the roof dome mounted destination blind (which also had underframe layout diferences from the 4-character route indicator vehicles).
The final number/letter pair - not carried on the vehicle in the image - defines variants within the diagram. By default there is a 'basic type', often 0A or 1A. In the case of DQ 212 the basic type with the standard equipment listed for the design was 1A (which had tungsten lighting), and there was a 2B variant for refurbished vehicles with fluorescent & tungsten lighting (vehicles that were fitted with fluorescent lights still had tungsten in the cab / vestibules / toilets).
Another important difference from the earlier diagrams is the use of issue numbers, with updated sheets carrying higher issue numbers. In the future I hope to add more of these different sheets here.
For more details on BR's diagram books can be found on the Barrowmore Model Railway Group website.