Gary,
You've been in this long enough and deeply enough to know that one feature, does not (in)authenticity make. If you have other information that you think may bear on an opinion, then feel free to drop me a PM.
A missing factory code can happen by at least two mechanisms. First, the stamper running dry and the worker not noticing. I'm guessing this would be likely and benign. Second, box accidentally bypassed the stamping. Again, not inconceivable and benign. Of course the less desirable mechanism would be that this is not an authentic box.
Provenance is key. In many cases, only this can serve as the final arbiter of authenticity.
Wilkey