I cannot find what the G means when it is used as the 2nd prefix of a body number.

ie YG2S1 YG2S2 etc. I know that the last number represents the model of car but I cannot find what the G means.

Where did you find that? I was looking through the Australian Mini and Moke book as well as other places and couldn't find it.

First prefix letter = Origin of Manufacture

Second prefix letter = Name
A= Austin
G= MG
H= Healey
M= Morris
W= Wolseley
K= Morris Cooper

Third prefix letter = (model) Cubic Capacity
L = up to 799cc
A = 800cc to 999cc
G = 1000cc to 1399cc
H = 1400cc to 1999cc
B = 2000cc to 2999cc
D = 3000cc up

4th prefix letter = Body type
N = 2 Seater Tourer
T = 4 Seater Tourer
W = Station Wagon
S = 4 Door Saloon
2S = 2 Door Saloon
X = Taxi

5th prefix letter = Series of Model. 2,3,etc. used to record a major change

So the YMA2S2 number means...
Y = Australia
M = Morris
A = 800cc to 999cc (in this case a 998cc)
2S = 2 Door Saloon (must be a mini!)
2 = Deluxe

Now the YG2S1
Y = Australia
G = 1000cc to 1399cc (in this case a 1098cc)
2S = 2 Door Saloon
1 = K

Yeah.. see that's why I am confused (or interested), because that page says that the 3rd prefix G is 1098cc - 1275cc.... and that the 2nd prefix G = MG. But further down they say that the G in YG2S1 is 1098cc - 1275cc.

Where did you find that? I was looking through the Australian Mini and Moke book as well as other places and couldn't find it.