Regimental numbers: letter prefixes

In 1920, the army introduced a new system of issuing numbers to its men and women. Each soldier would have a unique number, an army number. Before that, each regiment (and in some cases, the units in a regiment) issued its own numbers. This meant that when a man changed regiments, he usually had to change number. Some of those regimental numbering schemes included letter prefixes. These can offer useful clues.

tipTip: prefix? what prefix? Don't worry if your man did not have a number prefix: most did not.
But mine's not there! The list of prefixes below is almost certainly incomplete and is offered as general guidance only.

Prefix Regiments known to have used this prefix Notes
A Royal Sussex Regiment, King's Royal Rifle Corps  
AHT or A(HT) Army Service Corps Horsed Transport Special Reserve
B Rifle Brigade  
B Royal Fusiliers Bankers Battalion
BHT or B(HT) Army Service Corps Horsed Transport Special Reserve
C Royal Irish Rifles  
C King's Royal Rifle Corps 16th Bn (Church Lads Brigade)
CA Army Service Corps  
CMT or C(MT) Army Service Corps Mechanical Transport Special Reserve
D Honourable Artillery Company, Dragoons, Dragoon Guards, King Edward's Horse, Sussex Yeomanry, Hertfordshire Yeomanry  
DM2 Army Service Corps Mechanical Transport Learner
E Royal Fusiliers Used for 17th (Empire) Bn
E Army Service Corps Forage
F Royal Fusiliers, Middlesex Regiment 'Football' battalions
F Army Service Corps Forage (not paid from Army Funds)
G Many regiments General service
GS Many regiments General service
H Hussars regiments, North Irish Horse, Army Cyclist Corps  
H Army Service Corps  
J Royal Fusiliers, Middlesex Regiment Middlesex: this prefix was used for the 38th, 39th and 40th Bns (Jewish)
K Royal Fusiliers Kensington Battalion
L Lancers regiments  
L Royal Field Artillery Local enlistment
L Many infantry regiments  
M Honourable Artillery Company  
M Army Service Corps Mechanical Transport
M1 Army Service Corps Mechanical Transport
M2 Army Service Corps Mechanical Transport
MS Army Service Corps Mechanical Special
NAC Army Service Corps  
NCB Northern Cyclist Battalion  
O Rifle Brigade  
O Army Ordnance Corps  
P Rifle Brigade  
P Military Foot Police and Military Mounted Police  
PET Army Service Corps Petroleum Department
PS Royal Fusiliers, Middlesex Regiment Public Schools Battalions
PW Middlesex Regiment Public Works Battalion
R Many infantry regiments  
R Army Veterinary Corps  
R4 Army Service Corps Remounts
RS Army Service Corps Remounts Special
RTS Army Service Corps Remounts Special
RX4 Army Service Corps Remounts
S Many infantry regiments General service
S Army Service Corps Supply. Often accompanied with a number, denoting recruitment into the appropriately numbered Kitchener (New) Army
SD Royal Sussex Regiment South Downs Battalions
SE Army Veterinary Corps Special enlistment (General service)
SPTS Royal Fusiliers Sportsmen's Battalion
SR Many regiments  
SS Royal Field Artillery Shoeing Smith
SS Army Service Corps Supply Special
STK Royal Fusiliers Stockbroker's Battalion
T Many regiments Territorial
T Army Service Corps Horsed Transport. Often accompanied with a number, denoting recruitment into the appropriately numbered Kitchener (New) Army
TF Many regiments Territorial
TISR Army Service Corps  
TR Training Reserve  
TS Army Service Corps Transport Special
TSC Army Service Corps  
TT Army Veterinary Corps Territorial
W Royal Field Artillery Welsh
W Cheshire Regiment Wirral
W Military Provost Staff Corps  
WE Royal Army Medical Corps  
WT4 Army Service Corps Welsh Horsed Transport
Y King's Royal Rifle Corps Special Reserve
Z Rifle Brigade Special Reserve