Ya just gotta trust who ya got it from 
Only marking will be on the floorplate.
Only marking will be on the floorplate.

Cause... cause it says Colt on it! Ha!So why get a colt mag?
I agree with the others. You may want to return it; no marking most likely = made in who-knows-where junk.I just purchased a new colt 20rd mag for my AR. There are no markings on this magazine, inside or out. how can i be sure this is genuine colt mag?