There is some excellent details about German WWI manufacturers marks on bullets in the ammunition section of Dieter Storz's superb book on the Mauser 98.
Returning to the British bullet base markings, the list is:
1 Unknown contractor No.1 (probably Eley Bros) on round nosed bullets
1 Government Cartridge Factory No.1 (Mark VII bullets)
2 Unknown contractor No.2 (Round nosed bullets)
3 Unknown contractor No.3 (Round nosed bullets)
3 Government Cartridge Factory No.3 (Mark VII bullets)
6 Unknown contractor No.6
B Birmingham Metals and Munitions Co.
CP Crompton Parkinson
D Dum Dum Arsenal
E Eley Bros.
^ Royal Laboratory, Woolwich. Also found with * or other mark below Broad Arrow)
G Greenwood & Batley Ltd
GB Greenwood & Batley Ltd (usually with "VII" underneath)
K Kynoch
O Royal Laboratory, Woolwich
N Dum Dum, Northern Circuit, sometimes in mirror image (Round nosed bullets)
N Nobel Explosives Ltd, Glasgow (Mark VII bullets)
R R.O.F. Radway Green
RW Rudge-Whitworth Cycle Co.
S Dum Dum Southern Circuit
U Reminfgton UMC.
There may be others but I have not seen them.
Also, don't forget that the insides of most WW2 and later .303 cases have the initial of the factory that made the case stamped between the fire holes. This is also true of .50 Vickers cases.
Regards
TonyE
