Brownells "Bench Talk" lists the serial number cut-offs for pre-post ban for most AR-15 makers. However, for Bushmaster, they list their numbers as being very jumbled and you must contact them to find out for sure if a particular number is pre or post. That is why you got the answer you did from them, and that is also why you cannot take to the bank that a serial number before yours being post automatically makes yours post. They must have a very confused operation over there.