George asks from the USA:
Can you enlighten me as to the purpose of the large number board over the end windows that this class of car as well as similar number boards that so many other MU's and diesels have.
Photos always seem to show a combination of Number-Letter-Number-Number but I cannot find a site that explains what these mean. A photo of a DMU117 in a museum setting shows it with "2K31".
The answer is that those headcodes are for train reporting.
I can't do any better than point you at this website: http://www.2d53.co.uk/Headcode/Headcode4.htm
From which we can decode the headcode in the photo to mean:
1 - Express passenger train, postal train, newspaper train or breakdown train or
snow plough going to clear the line, or light locomotive going to assist
disabled train. Officers' special train not requiring to stop in section.
O- Inter-regional train heading towards the Southern
42 - The route indication. I've looked but can't find out which specific route this is. (Update: See the Comments for more on this)
When these codes ceased to be used at the end of 1975, train crews were supposed to set the blinds (the numbers are on separate fabric strips like bus blinds) to four 0's. Most did but occasionally you'll see more adventurous versions with the loco number or even class being shown instead.
Later, the blinds were replaced by a pair of white dots or the headcode box was plated over with metal.