Hi everyone,
I went through my documentation to find out what the codes are on US ordnance. I found this small list that I must have copied from some document in the past, but can't remember where it came from.
I think this represents mainly the SAA cartridges, but maybe it will help to identify the stuff you got.
Here it is:

D A -Dominion Arsenal
D-18 -Dominion Arsenal (1918 tracer)
D I -Defense Industries Ltd., Canada
D E N -Denver Ordnance Plant
E C -Evansville-Chrysler (.45 only)
E C S -Evansville-Chrysler-Sunbeam (.45 only)
E W -Eau Claire Ordnance Plant
F A -Frankford Arsenal
F M -Fabrico Nacional de Municiones, Mexico
G E -General Electric Company
K S -Allengheny Oednance Plant (.50 only)
L C -Lake City Ordnance Plant
L M -Lowell Ordnance Plant (.50 only)\\
M -Milwaukee Ordnance Plant (.50 only)
P C -King Mills Ordnance Plant
P C C -Peters Cartridge Company
Q A -Dominion Arsenal, Quebec
R A -Remington Arms Company
T R -Three Rivers, Quebec
T W -Twin Cities Ordnance Plant
U -Utah Ordnance Plant
U Y -Utah Ordnance Plant
W -Western Cartridge Company
W 18 -Western Cartridge Company
W C W C C -Western Cartridge Company
W R A -Winchester Reapeating Arms Company

Maybe someone has other codes and abbrevations to add to this list?
Cheers, Lex