Back in 1998, AR-15.com had a serial number list that had a Bushmaster serial number page as follows:
Below L051000 - Assembled Pre-Ban Rifles
L051001-L063000 - Mixed Pre-Ban Assembled Rifles and Pre-Sept 94 Lowers (Call to verify)
L063001 and Up - Post-Ban.
I am not sure why they took this information off? But they had a large disclaimer and suggested you need to verify the number with Bushmaster. It only takes a minute... 1-800-998-SWAT. It looks Pre-Ban but I'd call to be sure.