Diesel loco headcode discs, another question.

[sb67]

I'm adding headcode discs to my class 31. In doing my research I've noticed when the discs are shut they are either down or up, where the hole in the disc shows a circle of white behind it. 

Does anyone know if there was a reason for the disc being shut either way? 

 

Many thanks.

[The Johnster]

The discs were displayed in the same way as lamps on a steam loco, the pattern displayed being the headcode (two over tge buffers, class 1 express passenger, one on too the smokebox, class 2 ordinary passenger and do on; google BR steam locomotive headlamp codes).  With the lamps lit at night the effect was identical, but whereas the signalman can see the lamps by daylight even when they are not lit,  meaning he can see the headcode and class of train, on a diesel with discs only those required to display the headcode are ‘open’, while the ones not required are shut.  In this way the signalman can read the class of train by the headcode.  
 

Superceded by 4-character headcode blinds lit at night,, the first character denoting the class of train.  

[Izzy]

I’ve noticed this particularly with diesels on exGE lines in the London areas.  At first I thought it was about route indication as with them ‘up’ rather than down they then display that little white dot through the hole. But I have found no information to support this theory. I do think having them up rather than down meant at night the lights could be used without having to change the disc position in the dark.

 

Bob

[russ p]

They were supposed to folded down to cover the lamp , I believe the reason that they also folded up was to enable the bulb to be changed in the lamp 

[sb67]

Thank you guys. I didn't know if they were showing the white dot for a reason. I have seen plenty of photos with the disc closed but in the up position, showing a white dot. Thankfully I've also seen some of the loco's I want to model with them in the down position covering the lamp, so that will be the easiest option, I think.

 

Next question. Can anyone recommend any 4mm scale discs as the Shawplan ones are a bit small to cover the lamps when closed?