Pics are always helpful in these instances, but it sounds like you have a National Postal Meter M1 carbine. It should also be marked "U.S. Carbine, Cal. 30 M1" on the top of the receiver, at the breech. Check the barrel, just below the step in the diameter, behind the rear sight. There may be a manufacturer name and date there. Carefully examining all the parts will turn up manufacturing codes on all the parts with an area big enough to stamp - lol. There is a lot of info on the 'net on the M1 carbine, so just fire up your favourite search engine.