Either your potential buyer doesn't know what he's talking about, or he's trying to feed you a line of steer manure. A parts gun would still have both the "AL" marking and the serial number on the receiver. Unless the serial number or "AL" marking were purposely altered, the serial number wouldn't have any bearing on whether or not it's a parts gun. Also, there are many known Luxembourg contract rifles with serial numbers much lower than yours. My personal "AL" rifle has a serial number of 22xx and I've read of them as low as 19xx, so yours certainly isn't too low to be a Luxembourg contract. One note, both the receiver and receiver cover should have the same serial number stamped on the left side.
Just out of curiosity, did your potential buyer try to buy your rifle at a reduced price?