The GC diagram numbers were of the form Number Letter Number, where the first number indicated the type of vehicle:
1 - NPCS
2 - Four and Six wheeled passenger coaches
3 - Ordinary bogie coaches
4 - Ordinary bogie coaches with Lavatories
5 - Gangwayed bogie coaches
6 - Railmotors etc.
The letter indicated the type of accommodation:
A - Van Third
B - Third
C - Open Third
D - Restaurant Third
E - Open Van Third
F - Saloon Third
G - Open Third, also Kitchen Car*
H - Van Composite
J - Slip Composite
K - Baggage Composite
L - Composite
M - Restaurant Composite
N - Saloon Composite
P - Van First
Q - Ordinary First
R - Restaurant First
S - Saloon First
T - Open Carriage Truck
U - Covered Carriage Truck
W - Special Cattle Box
X - Horse Box
Y - Brake Van
Z - Miscellaneous Van
* The Barnum saloons were 5G1, composite restaurants that were rebuilt
as kitchen cars by the LNER became 5G2 etc.
The final digits of diagram number was a serial of the initial number
and letter, unfortunately these don't align with the various body
styles, for instance, the sequence of lavatory composites is:
4L1 - 38' MS&L built 1880
4L2 to 4L6 - clerestories built 1903/4
4L7 & 4L8 - London suburban built 1906 & 1907
4L9 & 4L10 - 60' matchboard built 1911/2 & 1920
I have a number of diagrams, though not a full book, but if you need
help identifying carriages let me know and I'll see what I can do.