I got this information from William Cohenour's book
Mint Marks Are:
CC---Continental Coin Corp (Closed)
CT---Casino Tokens (Closed)
G----Global Minting
GDC--Green Duck Corporation (Closed)
LM---Letcher Mint (Acquired By Global Minting)
NCM--Nevada Coin Mart ( Became Nevada Coin Minting)
RWM--Roger Williams Mint
S----Sunshine Mint
TPM--The Perth Mint