most pre bans have the bayonet lug
there was really no reason to remove them pre 1994
a threaded barrel is another preban attribute that is easy to spot
heres the definition:
Any semiautomatic rifle that has the ability to accept a detachable magazine and has at least 2 of the following features:
* a folding or telescoping stock.
* a pistol grip that protrudes conspicuously beneath the action of the weapon.
* a bayonet mount.
* a flash suppressor or a threaded barrel designed to accommodate a flash suppressor.
* a grenade launcher.
http://old.ar15.com/legal/preORpost.asp
http://old.ar15.com/legal/serialNumberList.asp
Colt 800-962-COLT
CC001616 and Below - Pre-Ban
CH019500 and Below - Pre-Ban
GC018500 and Below - Pre-Ban
LH011326 and Below - Pre-Ban
MH086020 and Below - Pre-Ban
NL004800 and Below - Pre-Ban
SL027246 and Below - Pre-Ban
SP360200 and Below - Pre-Ban
1963 SP00001-SP00023**
1964 SP00101
1965 SP02501
1966 SP05600
1967 SP08250
1968 SP10750
1969 SP14000-SP14653
1970 SP15001-SP15473
1971 SP16001
1972 SP19401
1973 SP24201
1974 SP32601
1975 SP43801
1976 SP55301
1977 SP67651
1978 SP83400
ST038100 and Below - Pre-Ban
TA10100 and Below - Pre-Ban
BD000134 and Below - Pre-Ban (AR-15A3 Tactical Carbine - very rare model)
MT00001 and Above - Post-Ban
BK000001and Above - Post Ban (CAR-A3)
CST000001 and Above - Post-Ban (Began Late 1997 Production)
CMH000001 and Above - Post Ban
CCH010000 and Above - Post Ban MT6700, MT6700C and MT6700CH series, 1998 production