Pre-ban Status of a gun depends on when it was made AND the configuration of the gun at some point in time before the ban.
Pre-ban--made before Sept. 1994, has a pistol grip, detachable mag, and one or more of the following features: Collapsable Stock, Bayo Lug, Flash Hider. The way the law is written, this gun is "grandfathered"--i.e. all assault weapons are illegal with the exception of those existing in assault weapon form before Sept. 1994.
Post-ban--made after Sept. 1994, has a pistol grip and detachable mag, therefore it cannot have any of the other evil features. If it is like an M-1A Springfield, it could have a flash hider but no pistol grip (or bayo lug).
***Post-ban--Made before Sept. 1994 but NOT IN ASSAULT WEAPON CONFIGURATION before that date. If you bought a "factory" Bushmaster Varminter type gun (or V-match) with a pistol grip and detachable mag that was made in 1993 (hbar, no flash hider, bayo lug or collapsable stock), it would still be a POST-BAN gun, because it didn't have the evil features installed before the ban.
***Post-ban--Stripped "virgin" receivers made before Sept. 1994 that were never assembled into firearms. The exception is that if you have all the parts, it is considered "assembled" by the ATF, even if you did not put it together. So if I bought a Bushmaster stripped lower in May, 1994 AND had a Commando (11.5" bbl, 5.5" flash hider) upper and pistol grip in my possesion as of Sept. 1994 (and I better keep receipts), it is a pre-ban receiver.
So, for the M-1 Carbine, it is a post ban (not grandfathered) gun, unless it had a pistol grip (assuming in it's standard configuration, it has a bayo lug and detachable mag) OR flash hider installed sometime before Sept. 1994. If I recall properly, the M-1 Carbine can have a flash hider that just snaps (or is held with screws) on to the front sight.
Interesting aside--at one point, the ATF, in their infinite stupidity, was trying to claim that a rifle must ALWAYS be in assault rifle configuration to remain pre-ban. So, in the pre-ban cases above, if I sold the upper (pre ban features), and installed a target upper (no pre-ban features), it would magically go from being a pre-ban to a post-ban gun. Then, if I reinstalled a pre-ban upper, it would magically go from being a post-ban sporting rifle to an illegal assault weapon.
For all practical purposes, I figure it this way--you might be up Schitz Creek if you have a gun that is built on a receiver purchased early September 1994 (right before the ban) and you cannot show that you had the remaining assault weapon parts at the same time. That is assuming the ATF wants to hassle you (generally for something else at the same time). You would probably be OK if you bought a stripped receiver in 1990 but didn't have receipts to show that you had all the other parts in hand before 1994 (regardless of the law saying you need to prove it was an assault weapon, not the government needing to prove it). An M-1 Carbine, made 15+ years before the ban?? I think you would have almost no trouble in any configuration.