It would be unusual for an alexa not to have a serial number as from memory, the alexa was produced after the short window when there were no numbers. Otherwise, the bag does appear to be authentic and if it really doesn't have a number, it could just be that the maker forgot to add it. That happened with a batch of somerset models where they are missing the number. However, unless you are particularly set on that bag, I would wait for another to come along that has a number, as not having one could cause concern for any future buyer when you come to sell it.