My brother had a Mini 14 years ago, and just now got rid of the rifle. He had aquired about 600 rounds of 5.56 for the rifle, and I was wondering what the head stamps are. He has several older white boxes that say American and the head stamp is WCC 89 or WCC 90. The other rounds are on stripper clips, and the head stamp reads TW 73. I believe the first head stamps are older Q3131, but I have no Idea what the TW 73 is.
CartridgeCollectors.org has a good section on Headstamp Codes:
TW > Twin Cities Ordnance Plant, Minneapolis, MN
and from Wartime Ammo Producers in the USA by Jim Frigiola:
Twin Cities Ordnance Plant (TW)
Located near Minneapolis, MN it was another of the first wave ordnance plants, and was operated by Federal Cartridge Co. Twin Cities operated from 1941 to 1945, from 1951 to 1957, and again from 1966 to 1975. One of the largest volume producers, Twin Cities made caliber 30, caliber 45, caliber 50, caliber 50 spotter-tracer, 7.62mm and 5.56mm ammunition.