Below L051000 - Assembled Pre-Ban Rifles and some pre-94 lowers.**
L051001-L063000 - Mixed Pre-Ban Assembled Rifles and Pre-Sept 94 Lowers (Call to verify)
L063001 and Up - Post-Ban
As mentioned above, a pre-ban serial is not necessarily proof of pre-ban assembly and grandfathered status. If you can get a letter from the seller attesting to pre-ban assembly before 9/13/94, that would be enough proof to satisfy me, personally.
Any lower with a serial higher than 63000 is definitely post-ban, though.