The more I explore this topic, the more things get "curioser and curioser", to quote Alice.
When you look at the .TgPcDx files as
images (I used the RW_Tools program to do this), you can see that all 26 letters have been assigned a display in a .TgPcDx file. Here are a couple of examples.
Some of the files include two different items, in which the first is shown in larger letters than the second. An example of this is the
destination_a.TgPcDx file produces this image ...
Other files contain a single item, which is shown in the larger letters. An example of this is the
destination_g.TgPcDx file, which produces this image ...
Here are all the assignments. When there are two things listed, the items shown inside the parentheses are displayed in smaller type on the second line, as the "All Stations" is shown in the first example above). When there is a single item, it is shown in larger letters, as shown in the second example above.
a ... Banbury (All Stations)
b ... Basingstoke
c ... Cheltenham (Spa)
d ... Didcot
e ... Gloucester (Express)
f ... Greenford (First Great Western)
g ... Gatwick Apt
h ... Hereford
i ... Great Malvern
j ... Henley (On Thames)
k ... Marlow
l ... Paddington (FGW Link)
m ... Maidenhead
n ... Newbury (Not in Service)
o ... Oxford
p ... Portsmouth (Limited Stop)
q ... Bedwyn (via Oxford/Worcester)
r ... Reading
s ... Slough (Network Southeast)
t ... Shalford (Thames Trains)
u ... Guildford (Chiltern Trains)
v ... Windsor + Eton (Central)
w ... Worcester (Shrub Hill)
x ... Sorry (Charter)
y ... Class 166 (Class 166)
z ... Special (Special)
As I mentioned before, it seems as if somebody went to a lot of trouble to do all this. And yet I can't find any documentation, from either before or after upgrade, to describe the Class 166 at all!